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Neuroscience | Define Neuroscience at Dictionary.com

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition William Collins Sons & Co. Ltd. 1979, 1986 HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012 Cite This Source

neuroscience neuroscience (nur'-s'ns, nyur'-) n. Any of the sciences, such as neuroanatomy and neurobiology, that deal with the nervous system.

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Immunology – Wikipedia, the free encyclopedia

Immunology is a branch of biomedical science that covers the study of all aspects of the immune system in all organisms.[1] It deals with the physiological functioning of the immune system in states of both health and diseases; malfunctions of the immune system in immunological disorders (autoimmune diseases, hypersensitivities, immune deficiency, transplant rejection); the physical, chemical and physiological characteristics of the components of the immune system in vitro, in situ and in vivo. Immunology has applications in several disciplines of science, and as such is further divided.

Even before the concept of immunity (from immunis, Latin for "exempt") was developed, numerous early physicians characterized organs that would later prove to be part of the immune system. The key primary lymphoid organs of the immune system are the thymus and bone marrow, and secondary lymphatic tissues such as spleen, tonsils, lymph vessels, lymph nodes, adenoids, and skin and liver. When health conditions warrant, immune system organs including the thymus, spleen, portions of bone marrow, lymph nodes and secondary lymphatic tissues can be surgically excised for examination while patients are still alive.

Many components of the immune system are actually cellular in nature and not associated with any specific organ but rather are embedded or circulating in various tissues located throughout the body.

Classical immunology ties in with the fields of epidemiology and medicine. It studies the relationship between the body systems, pathogens, and immunity. The earliest written mention of immunity can be traced back to the plague of Athens in 430 BCE. Thucydides noted that people who had recovered from a previous bout of the disease could nurse the sick without contracting the illness a second time. Many other ancient societies have references to this phenomenon, but it was not until the 19th and 20th centuries before the concept developed into scientific theory.

The study of the molecular and cellular components that comprise the immune system, including their function and interaction, is the central science of immunology. The immune system has been divided into a more primitive innate immune system and, in vertebrates, an acquired or adaptive immune system. The latter is further divided into humoral (or antibody) and cell-mediated components.

The humoral (antibody) response is defined as the interaction between antibodies and antigens. Antibodies are specific proteins released from a certain class of immune cells known as Blymphocytes, while antigens are defined as anything that elicits the generation of antibodies ("anti"body "gen"erators). Immunology rests on an understanding of the properties of these two biological entities and the cellular response to both.

Immunological research continues to become more specialized, pursuing non-classical models of immunity and functions of cells, organs and systems not previously associated with the immune system (Yemeserach 2010).

Clinical immunology is the study of diseases caused by disorders of the immune system (failure, aberrant action, and malignant growth of the cellular elements of the system). It also involves diseases of other systems, where immune reactions play a part in the pathology and clinical features.

The diseases caused by disorders of the immune system fall into two broad categories:

Other immune system disorders include various hypersensitivities (such as in asthma and other allergies) that respond inappropriately to otherwise harmless compounds.

The most well-known disease that affects the immune system itself is AIDS, an immunodeficiency characterized by the suppression of CD4+ ("helper") T cells, dendritic cells and macrophages by the Human Immunodeficiency Virus (HIV).

Clinical immunologists also study ways to prevent the immune system's attempts to destroy allografts (transplant rejection).

The bodys capability to react to antigen depends on a person's age, antigen type, maternal factors and the area where the antigen is presented.[2]Neonates are said to be in a state of physiological immunodeficiency, because both their innate and adaptive immunological responses are greatly suppressed. Once born, a childs immune system responds favorably to protein antigens while not as well to glycoproteins and polysaccharides. In fact, many of the infections acquired by neonates are caused by low virulence organisms like Staphylococcus and Pseudomonas. In neonates, opsonic activity and the ability to activate the complement cascade is very limited. For example, the mean level of C3 in a newborn is approximately 65% of that found in the adult. Phagocytic activity is also greatly impaired in newborns. This is due to lower opsonic activity, as well as diminished up-regulation of integrin and selectin receptors, which limit the ability of neutrophils to interact with adhesion molecules in the endothelium. Their monocytes are slow and have a reduced ATP production, which also limits the newborn's phagocytic activity. Although, the number of total lymphocytes is significantly higher than in adults, the cellular and humoral immunity is also impaired. Antigen-presenting cells in newborns have a reduced capability to activate Tcells. Also, Tcells of a newborn proliferate poorly and produce very small amounts of cytokines like IL-2, IL-4, IL-5, IL-12, and IFN-g which limits their capacity to activate the humoral response as well as the phagocitic activity of macrophage. Bcells develop early during gestation but are not fully active.[3]

Maternal factors also play a role in the bodys immune response. At birth, most of the immunoglobulin present is maternal IgG. Because IgM, IgD, IgE and IgA dont cross the placenta, they are almost undetectable at birth. Some IgA is provided by breast milk. These passively-acquired antibodies can protect the newborn for up to 18 months, but their response is usually short-lived and of low affinity.[3] These antibodies can also produce a negative response. If a child is exposed to the antibody for a particular antigen before being exposed to the antigen itself then the child will produce a dampened response. Passively acquired maternal antibodies can suppress the antibody response to active immunization. Similarly the response of T-cells to vaccination differs in children compared to adults, and vaccines that induce Th1 responses in adults do not readily elicit these same responses in neonates.[3] Between six to nine months after birth, a childs immune system begins to respond more strongly to glycoproteins, but there is usually no marked improvement in their response to polysaccharides until they are at least one year old. This can be the reason for distinct time frames found in vaccination schedules.[4][5]

During adolescence, the human body undergoes various physical, physiological and immunological changes triggered and mediated by hormones, of which the most significant in females is 17--oestradiol (an oestrogen) and, in males, is testosterone. Oestradiol usually begins to act around the age of 10 and testosterone some months later.[6] There is evidence that these steroids act directly not only on the primary and secondary sexual characteristics but also have an effect on the development and regulation of the immune system,[7] including an increased risk in developing pubescent and post-pubescent autoimmunity.[8] There is also some evidence that cell surface receptors on B cells and macrophages may detect sex hormones in the system.[9]

The female sex hormone 17--oestradiol has been shown to regulate the level of immunological response,[10] while some male androgens such as testosterone seem to suppress the stress response to infection. Other androgens, however, such as DHEA, increase immune response.[11] As in females, the male sex hormones seem to have more control of the immune system during puberty and post-puberty than during the rest of a male's adult life.

Physical changes during puberty such as thymic involution also affect immunological response.[12]

The use of immune system components to treat a disease or disorder is known as immunotherapy. Immunotherapy is most commonly used in the context of the treatment of cancers together with chemotherapy (drugs) and radiotherapy (radiation). However, immunotherapy is also often used in the immunosuppressed (such as HIV patients) and people suffering from other immune deficiencies or autoimmune diseases. Like IL2,IL10,GM-CSF B,INF a .

The specificity of the bond between antibody and antigen has made it an excellent tool in the detection of substances in a variety of diagnostic techniques. Antibodies specific for a desired antigen can be conjugated with an isotopic (radio) or fluorescent label or with a color-forming enzyme in order to detect it. However, the similarity between some antigens can lead to false positives and other errors in such tests by antibodies cross-reacting with antigens that aren't exact matches.[13]

The study of the interaction of the immune system with cancer cells can lead to diagnostic tests and therapies with which to find and fight cancer.

This area of the immunology is devoted to the study of immunological aspects of the reproductive process including fetus acceptance. The term has also been used by fertility clinics to address fertility problems, recurrent miscarriages, premature deliveries and dangerous complications such as pre-eclampsia.

Immunology is strongly experimental in everyday practice but is also characterized by an ongoing theoretical attitude. Many theories have been suggested in immunology from the end of the nineteenth century up to the present time. The end of the 19th century and the beginning of the 20th century saw a battle between "cellular" and "humoral" theories of immunity. According to the cellular theory of immunity, represented in particular by Elie Metchnikoff, it was cells more precisely, phagocytes that were responsible for immune responses. In contrast, the humoral theory of immunity, held, among others, by Robert Koch and Emil von Behring, stated that the active immune agents were soluble components (molecules) found in the organisms humors rather than its cells.[14][15][16]

In the mid-1950s, Frank Burnet, inspired by a suggestion made by Niels Jerne,[17] formulated the clonal selection theory (CST) of immunity.[18] On the basis of CST, Burnet developed a theory of how an immune response is triggered according to the self/nonself distinction: "self" constituents (constituents of the body) do not trigger destructive immune responses, while "nonself" entities (pathogens, an allograft) trigger a destructive immune response.[19] The theory was later modified to reflect new discoveries regarding histocompatibility or the complex "two-signal" activation of T cells.[20] The self/nonself theory of immunity and the self/nonself vocabulary have been criticized,[16][21][22] but remain very influential.[23][24]

More recently, several theoretical frameworks have been suggested in immunology, including "autopoietic" views,[25] "cognitive immune" views,[26] the "danger model" (or "danger theory",[21] and the "discontinuity" theory.[27][28] The danger model, suggested by Polly Matzinger and colleagues, has been very influential, arousing many comments and discussions.[29][30][31][32]

According to the American Academy of Allergy, Asthma, and Immunology (AAAAI), "an immunologist is a research scientist who investigates the immune system of vertebrates (including the human immune system). Immunologists include research scientists (PhDs) who work in laboratories. Immunologists also include physicians who, for example, treat patients with immune system disorders. Some immunologists are physician-scientists who combine laboratory research with patient care."[33]

Bioscience is the overall major in which undergraduate students who are interested in general well-being take in college. Immunology is a branch of bioscience for undergraduate programs but the major gets specified as students move on for graduate program in immunology. The aim of immunology is to study the health of humans and animals through effective yet consistent research, (AAAAI, 2013).[34] The most important thing about being immunologists is the research because it is the biggest portion of their jobs.[35]

Most graduate immunology schools follow the AAI courses immunology which are offered throughout numerous schools in the United States.[36] For example, in New York State, there are several universities that offer the AAI courses immunology: Albany Medical College, Cornell University, Icahn School of Medicine at Mount Sinai, New York University Langone Medical Center, University at Albany (SUNY), University at Buffalo (SUNY), University of Rochester Medical Center and Upstate Medical University (SUNY). The AAI immunology courses include an Introductory Course and an Advance Course.[37]The Introductory Course is a course that gives students an overview of the basics of immunology.

In addition, this Introductory Course gives students more information to complement general biology or science training. It also has two different parts: Part I is an introduction to the basic principles of immunology and Part II is a clinically-oriented lecture series. On the other hand, the Advanced Course is another course for those who are willing to expand or update their understanding of immunology. It is advised for students who want to attend the Advanced Course to have a background of the principles of immunology.[38] Most schools require students to take electives in other to complete their degrees. A Masters degree requires two years of study following the attainment of a bachelor's degree. For a doctoral programme it is required to take two additional years of study.[39]

The expectation of occupational growth in immunology is an increase of 36 percent from 2010 to 2020.[40] The median annual wage was $76,700 in May 2010. However, the lowest 10 percent of immunologists earned less than $41,560, and the top 10 percent earned more than $142,800, (Bureau of Labor Statistics, 2013). The practice of immunology itself is not specified by the U.S. Department of Labor but it belongs to the practice of life science in general.[41]

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Immunology - Wikipedia, the free encyclopedia

The American Academy of Allergy Asthma and Immunology | AAAAI

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Patients and healthcare professionals depend on authoritative, expert resources to make decisions about allergies, asthma and immune deficiency disorders.

Welcome to the AAAAI's website, a membership organization of more than 6,800 allergist / immunologists and related professionals world-wide with advanced training and experience dedicated to allergy, asthma, immune deficiencies & immunologic diseases.

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Trinity Allergy, Asthma, and Immunology Care

Trinity Allergy, Asthma and Immunology Care, P.C. is a professional corporation devoted to helping patients with allergy, asthma and primary immunodeficiency diseases. Trinity is located in Kingman, Arizona about 100 miles south of Las Vegas on the campus of Kingman Regional Medical Center. Trinity hopes to fulfill the healthcare needs of both adults and children residing in Mohave county. Trinity has two-satellite offices- one in Bullhead City to serve local residents and residents of Laughlin, Nevada and Needles, California and another one in Lake Havasu City to serve local residents and residents of Parker, Arizona and Needles, California. Dr. Natarajan Asokan, the founder and president of the corporation is a Board Certified Allergist & Immunologist and with excellent qualifications and extensive training. At Trinity, we believe in delivering high quality, state-of-the-art, compassionate and technologically sound healthcare services. If you, or your loved one, are suffering from allergy, asthma, or primary immunodeficiency diseases, we are confident that we can improve the quality of your life and health. On behalf of Dr. Asokan & his staff at Trinity, We thank you for your time and patience! Hope to serve your needs soon!!

Download what you need to know about Influenza and Swine Flu

3/5/2010: To accommodate the growing needs of residents of Bullhead City, Trinity moved its location in Bullhead City from 1975 Highway 95 to a bigger and better location next-door at 1971 Highway 95. The newer facility has a larger waiting area, more number of examination rooms and separate testing and injection areas. Our staff and patients just love our new location.

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Trinity Allergy, Asthma, and Immunology Care

Human nature – Wikipedia, the free encyclopedia

Human nature refers to the distinguishing characteristicsincluding ways of thinking, feeling and actingwhich humans tend to have naturally, independently of the influence of culture. The questions of what these characteristics are, how fixed they are, and what causes them are amongst the oldest and most important questions in western philosophy. These questions have particularly important implications in ethics, politics, and theology. This is partly because human nature can be regarded as both a source of norms of conduct or ways of life, as well as presenting obstacles or constraints on living a good life. The complex implications of such questions are also dealt with in art and literature, while the multiple branches of the humanities together form an important domain of inquiry into human nature and into the question of what it is to be human.

The branches of contemporary science associated with the study of human nature include anthropology, sociology, sociobiology, and psychology, particularly evolutionary psychology, which studies sexual selection in human evolution, and developmental psychology. The "nature versus nurture" debate is a broadly inclusive and well-known instance of a discussion about human nature in the natural sciences.

The concept of nature as a standard by which to make judgments was a basic presupposition in Greek philosophy. Specifically, "almost all" classical philosophers accepted that a good human life is a life in accordance with nature.[1]

(Notions and concepts of human nature from China, Japan, or India are not taken up in the present discussion.)

On this subject, the approach of Socratessometimes considered to be a teleological approachcame to be dominant by late classical and medieval times. This approach understands human nature in terms of final and formal causes. Such understandings of human nature see this nature as an "idea", or "form" of a human.[2] By this account, human nature really causes humans to become what they become, and so it exists somehow independently of individual humans. This in turn has sometimes been understood as also showing a special connection between human nature and divinity.

However, the existence of this invariable human nature is a subject of much historical debate, continuing into modern times. Against this idea of a fixed human nature, the relative malleability of man has been argued especially strongly in recent centuriesfirstly by early modernists such as Thomas Hobbes and Jean-Jacques Rousseau. In Rousseau's Emile, or On Education, Rousseau wrote: "We do not know what our nature permits us to be."[citation needed] Since the early 19th century, thinkers such as Hegel, Marx, Kierkegaard, Nietzsche, Sartre, structuralists, and postmodernists have also sometimes argued against a fixed or innate human nature.

Still more recent scientific perspectivessuch as behaviorism, determinism, and the chemical model within modern psychiatry and psychologyclaim to be neutral regarding human nature. (As in all modern science, they seek to explain without recourse to metaphysical causation.) They can be offered to explain human nature's origins and underlying mechanisms, or to demonstrate capacities for change and diversity which would arguably violate the concept of a fixed human nature.

Philosophy in classical Greece is the ultimate origin of the western conception of the nature of a thing. According to Aristotle, the philosophical study of human nature itself originated with Socrates, who turned philosophy from study of the heavens to study of the human things.[3] Socrates is said to have studied the question of how a person should best live, but he left no written works. It is clear from the works of his students Plato and Xenophon, and also by what was said about him by Aristotle (Plato's student), that Socrates was a rationalist and believed that the best life and the life most suited to human nature involved reasoning. The Socratic school was the dominant surviving influence in philosophical discussion in the Middle Ages, amongst Islamic, Christian, and Jewish philosophers.

The human soul in the works of Plato and Aristotle has a divided nature, divided in a specifically human way. One part is specifically human and rational, and divided into a part which is rational on its own, and a spirited part which can understand reason. Other parts of the soul are home to desires or passions similar to those found in animals. In both Aristotle and Plato, spiritedness (thumos) is distinguished from the other passions (epithumiai).[4] The proper function of the "rational" was to rule the other parts of the soul, helped by spiritedness. By this account, using one's reason is the best way to live, and philosophers are the highest types of humans.

AristotlePlato's most famous studentmade some of the most famous and influential statements about human nature. In his works, apart from using a similar scheme of a divided human soul, some clear statements about human nature are made:

For Aristotle, reason is not only what is most special about humanity compared to other animals, but it is also what we were meant to achieve at our best. Much of Aristotle's description of human nature is still influential today. However, the particular teleological idea that humans are "meant" or intended to be something has become much less popular in modern times.[8]

For the Socratics, human nature, and all natures, are metaphysical concepts. Aristotle developed the standard presentation of this approach with his theory of four causes. Human nature is an example of a formal cause, according to Aristotle. Their teleological concept of nature is associated with humans having a divine component in their psyches, which is most properly exercised in the lifestyle of the philosopher, which is thereby also the happiest and least painful life.

One of the defining changes that occurred at the end of the Middle Ages was the end of the dominance of Aristotelian philosophy, and its replacement by a new approach to the study of nature, including human nature. In this approach, all attempts at conjecture about formal and final causes were rejected as useless speculation. Also, the term "law of nature" now applied to any regular and predictable pattern in nature, not literally a law made by a divine law-maker, and, in the same way, "human nature" became not a special metaphysical cause, but simply whatever can be said to be typical tendencies of humans.

Although this new realism applied to the study of human life from the beginningfor example, in Machiavelli's worksthe definitive argument for the final rejection of Aristotle was associated especially with Francis Bacon, and then Ren Descartes, whose new approach returned philosophy or science to its pre-Socratic focus upon non-human things. Thomas Hobbes, then Giambattista Vico, and David Hume all claimed to be the first to properly use a modern Baconian scientific approach to human things.

Hobbes famously followed Descartes in describing humanity as matter in motion, just like machines. He also very influentially described man's natural state (without science and artifice) as one where life would be "solitary, poor, nasty, brutish and short."[9] Following him, John Locke's philosophy of empiricism also saw human nature as a tabula rasa. In this view, the mind is at birth a "blank slate" without rules, so data are added, and rules for processing them are formed solely by our sensory experiences.[10]

Jean-Jacques Rousseau pushed the approach of Hobbes to an extreme and criticized it at the same time. He was a contemporary and acquaintance of Hume, writing before the French Revolution and long before Darwin and Freud. He shocked Western civilization with his Second Discourse by proposing that humans had once been solitary animals, without reason or language or communities, and had developed these things due to accidents of pre-history. (This proposal was also less famously made by Giambattista Vico.) In other words, Rousseau argued that human nature was not only not fixed, but not even approximately fixed compared to what had been assumed before him. Humans are political, and rational, and have language now, but originally they had none of these things.[11] This in turn implied that living under the management of human reason might not be a happy way to live at all, and perhaps there is no ideal way to live. Rousseau is also unusual in the extent to which he took the approach of Hobbes, asserting that primitive humans were not even naturally social. A civilized human is therefore not only imbalanced and unhappy because of the mismatch between civilized life and human nature, but unlike Hobbes, Rousseau also became well known for the suggestion that primitive humans had been happier, "noble savages".[12]

Rousseau's conception of human nature has been seen as the origin of many intellectual and political developments of the 19th and 20th centuries.[13] He was an important influence upon Kant, Hegel, and Marx, and the development of German idealism, historicism, and romanticism.

What human nature did entail, according to Rousseau and the other modernists of the 17th and 18th centuries, were animal-like passions that led humanity to develop language and reasoning, and more complex communities (or communities of any kind, according to Rousseau).

In contrast to Rousseau, David Hume was a critic of the oversimplifying and systematic approach of Hobbes, Rousseau, and some others whereby, for example, all human nature is assumed to be driven by variations of selfishness. Influenced by Hutcheson and Shaftesbury, he argued against oversimplification. On the one hand, he accepted that, for many political and economic subjects, people could be assumed to be driven by such simple selfishness, and he also wrote of some of the more social aspects of "human nature" as something which could be destroyed, for example if people did not associate in just societies. On the other hand, he rejected what he called the "paradox of the sceptics", saying that no politician could have invented words like "'honourable' and 'shameful,' 'lovely' and 'odious,' 'noble' and 'despicable,'" unless there was not some natural "original constitution of the mind."[14]

Humelike Rousseauwas controversial in his own time for his modernist approach, following the example of Bacon and Hobbes, of avoiding consideration of metaphysical explanations for any type of cause and effect. He was accused of being an atheist. He wrote:

We needn't push our researches so far as to ask "Why do we have humanity, i.e. a fellow-feeling with others?" It's enough that we experience this as a force in human nature. Our examination of causes must stop somewhere.[14]

After Rousseau and Hume, the nature of philosophy and science changed, branching into different disciplines and approaches, and the study of human nature changed accordingly. Rousseau's proposal that human nature is malleable became a major influence upon international revolutionary movements of various kinds, while Hume's approach has been more typical in Anglo-Saxon countries, including the United States.

As the sciences the concerned with humanity split up into more specialized branches, many of the key figures of this evolution expressed influential understandings about human nature.

Charles Darwin gave a widely accepted scientific argument for what Rousseau had already argued from a different direction, that humans and other animal species have no truly fixed nature, at least in the very long term. However, he also gave modern biology a new way of understanding how human nature does exist in a normal human time-frame, and how it is caused.

Sigmund Freud, the founder of psychoanalysis, famously referred to the hidden pathological character of typical human behavior. He believed that the Marxists were right to focus on what he called "the decisive influence which the economic circumstances of men have upon their intellectual, ethical and artistic attitudes." But he thought that the Marxist view of the class struggle was too shallow, assigning to recent centuries conflicts that were actually primordial. Behind the class struggle, according to Freud, there stands the struggle between father and son, between established clan leader and rebellious challenger. Freud also popularized his notions of the id and the desires associated with each supposed aspect of personality.

E. O. Wilson's sociobiology and closely related theory of evolutionary psychology give scientific arguments against the "tabula rasa" hypotheses of Hobbes, Locke, and Rousseau. In his book Consilience: The Unity of Knowledge (1998), Wilson claimed that it was time for a cooperation of all the sciences to explore human nature. He defined human nature as a collection of epigenetic rules: the genetic patterns of mental development. Cultural phenomena, rituals, etc. are products, not part of human nature. For example, artworks are not part of human nature, but our appreciation of art is. This art appreciation, or our fear for snakes, or incest taboo (Westermarck effect) can be studied by the methods of reductionism. Until now, these phenomena were only part of psychological, sociological, and anthropological studies. Wilson proposes that they can be part of interdisciplinary research.

An example of this fear is discussed in the book An Instinct for Dragons,[15] where anthropologist David E. Jones suggests a hypothesis that humans, just like other primates, have inherited instinctive reactions to snakes, large cats, and birds of prey. Folklore dragons have features that are combinations of these three, which would explain why dragons with similar features occur in stories from independent cultures on all continents. Other authors have suggested that, especially under the influence of drugs or in children's dreams, this instinct may give rise to fantasies and nightmares about dragons, snakes, spiders, etc., which makes these symbols popular in drug culture and in fairy tales for children. However, the traditional mainstream explanation to the folklore dragons does not rely on human instinct, but on the assumption that fossils of, for example, dinosaurs gave rise to similar fantasies all over the world.

In Christian theology, there are two ways of "conceiving human nature": (1) "spiritual, Biblical, and theistic" or (2) "natural, cosmical, and anti-theistic."[16] The focus in this section is on the first way. As William James put it in his study of human nature from a religious perspective, "religion" has a "department of human nature."[17]

Various views of human nature have been held by theologians. However, there are some "basic assertions" in all "biblical anthropology." (1) "Humankind has its origin in God, its creator." (2) "Humans bear the 'image of God'." (3) Humans are "to rule the rest of creation." (4) Humans have the "ability to transcend" themselves.[18]

The Bible contains no single "doctrine of human nature." Rather, it provides material for more philosophical descriptions of human nature.[19] For example, Creation as found in the Book of Genesis provides a theory on human nature.[20]

Created human nature As originally created, the Bible describes "two elements" in human nature: "the body and the breath or spirit of life breathed into it by God." By this was created a "living soul," that is, a "living person."[21] According to Genesis 1:27, this living person was made in the "image of God."[22] From the biblical perspective, "to be human is to bear the image of God."[23]

"Two main modes of conceiving human naturethe one of which is spiritual, Biblical, and theistic," and the other "natural, cosmical, and anti-theistic." John Tulloch[24]

Genesis does not elaborate the meaning of "the image of God," but scholars find suggestions. One is that being created in the image of God distinguishes human nature from that of the beasts.[25] Another is that as God is "able to make decisions and rule" so humans made in God's image are "able to make decisions and rule." A third is that mankind possesses an inherent ability to "to set goals" and move toward them.[26] That God denoted creation as "good" suggests that Adam was "created in the image of God, in righteousness."[27]

Adam was created with ability to make "right choices," but also with the ability to choose sin, by which he fell from righteousness into a state of "sin and depravity."[28] Thus, according to the Bible, "humankind is not as God created it."[29]

Fallen human nature By Adam's fall into sin, "human nature" became "corrupt." although it still bears [God's] image." The Bible, both the Old Testament and the New Testament, teaches that "sin is universal."[30] For example, "I was sinful at birth," says Psalm 51:5.[31] Jesus taught that everyone is a "sinner naturally" because it is mankind's "nature and disposition to sin."[32] Paul, in Romans 7:18, speaks of his "sinful nature."[33]

Such a "recognition that there is something wrong with the moral nature of man is found in all religions"[34]Augustine of Hippo coined a term for the assessment that all humans are born sinful: "original sin."[35] "Original sin" means "the tendency to sin innate in all human beings."[36]

"The corruption of original sin extends to every aspect of human nature": to "reason and will" as well as to "appetites and impulses." This condition is sometimes called "total depravity."[37] Total depravity does not mean that humanity is as "thoroughly depraved" as it could become.[38] Commenting on Romans 2:14, John Calvin writes that all people have "some notions of justice and rectitude . . . which are implanted by nature" all people.[39]

Adam embodied the "whole of human nature" so when Adam sinned "all of human nature sinned"[40] The Old Testament does not explicitly link the "corruption of human nature" to Adam's sin. However, the "universality of sin" implies a link to Adam. In the New Testament, Paul concurs with the "universality of sin." He also makes explicit what the Old Testament implied: the link between humanity's "sinful nature" and Adam's sin[41] In Romans 5:19, Paul writes, "through [Adam's] disobedience humanity became sinful."[42] Paul also applied humanity's sinful nature to himself: "there is nothing good in my sinful nature."[43]

The theological "doctrine of original sin" as an inherent element of human nature is not based only on the Bible. It is in part a "generalization from obvious facts" open to empirical observation.[44]

Empirical facts A number of experts on human nature have described the manifestations of original (i.e., the innate tendency to) sin as empirical facts.

Realistic view Liberal theologians in the early 20th century described "human nature" as "basically good" needing only "proper training and education." But the above examples document the return to a "more realistic view" of human nature "as basically sinful and self-centered." Human nature needs "to be regenerated . . . to be able to live the unselfish life."[51]

Regenerated human nature According to the Bible, "Adam's disobedience corrupted human nature" but God mercifully "regenerates."[52] "Regeneration is a radical change" that involves a "renewal of our [human] nature."[53] Thus, to counter original sin, Christianity purposes "a complete transformation of individuals" by Christ.[54]

The goal of Christ's coming is that fallen humanity might be "conformed to or transformed into the image of Christ who is the perfect image of God," as in 2 Corinthians 4:4.[55] The New Testament makes clear the "universal need" for regeneration.[56] A sampling of biblical portrayals of regenerating human nature and the behavioral results follow.

Total transformation The goal is to raise transform human nature to a "higher level" than "before the Fall," a level in which people will no longer sin.[61] Theology holds that this goal is not reached in this life where always "human nature is deficient." Thus, the goal cannot be reached until after one dies.[62]

There are two portrayals of reaching the goal in an intermediate state between earth and heaven: one Protestant, the other Roman Catholic.

A Protestant portrayal Randy Alcorn says that "In heaven there will be no evil desires or corruption." Thus, because everyone's "human nature is deficient," no one can go to heaven immediately after death. Between death and the "final destination" (a sinless "New Earth"), there is an intermediate state: a temporary "intermediate Heaven." In the resurrection from this temporary state, "human nature" will be restored to "the state of its ultimate perfection." The resurrected inhabitants of the final Heaven will be "so constituted or reconstituted" that they cannot sin because they do not want to.[63]

A Roman Catholic portrayal Jerry L. Walls agrees with the Protestant view that sinful human nature requires a "complete transformation" before admission into heaven. Earthly "regeneration begins this transformation" but does not complete it. Thus, as with the Protestant perspective, an intermediate state between earth and heaven is required. In Catholic doctrine, the intermediate state is called Purgatory. Purgatory is a "process of transformation" to fit its residents for heaven.[64]

Goal met In both views, the Christian goal of the transformation of sinful human nature will be met. The goal is that people will be "conformed to or transformed into the image of Christ who is the perfect image of God."[65]

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Human nature - Wikipedia, the free encyclopedia

Human sexual activity – Wikipedia, the free encyclopedia

This article is about sexual practices and related social aspects. For broader aspects of sexual behaviour, see Human sexuality. "Sexual activity" and "sexual behavior" redirect here. For sexual activity among other animals, see Animal sexual behaviour.

Human sexual activity, human sexual practice or human sexual behavior is the manner in which humans experience and express their sexuality. People engage in a variety of sexual acts from time to time, for a wide variety of reasons. Sexual activity normally results in sexual arousal and physiological changes in the aroused person, some of which are pronounced while others are more subtle. Sexual activity may also include conduct and activities which are intended to arouse the sexual interest of another, such as strategies to find or attract partners (courtship and display behavior), or personal interactions between individuals (for instance, foreplay). Sexual activity may follow sexual arousal.

Human sexual activity has sociological, cognitive, emotional, behavioral and biological aspects; these include personal bonding, sharing emotions and the physiology of the reproductive system, sex drive, sexual intercourse and sexual behavior in all its forms.

In some cultures, sexual activity is considered acceptable only within marriage, while premarital and extramarital sex are taboo. Some sexual activities are illegal either universally or in some countries, while some are considered contrary to the norms of a society. Two examples that are criminal offenses in most jurisdictions are sexual assault and sexual activity with a person below the local age of consent.

Sexual activity can be classified in a number of ways. It can be divided into acts which involve one person, also called autoeroticism, such as masturbation, or two or more people such as vaginal sex, anal sex, oral sex or mutual masturbation. If there are more than two participants in the sex act, it may be referred to as group sex. Autoerotic sexual activity can involve use of dildos, vibrators, anal beads, and other sex toys, though these devices can also be used with a partner.

Sexual activity can be classified into the gender and sexual orientation of the participants, as well as by the relationship of the participants. For example, the relationships can be ones of marriage, intimate partners, casual sex partners or anonymous. Sexual activity can be regarded as conventional or as alternative, involving, for example, fetishism, paraphilia, or BDSM activities.[1][2] Fetishism can take many forms ranging from the desire for certain body parts, for example large breasts, armpits or foot worship. The object of desire can often be shoes, boots, lingerie, clothing, leather or rubber items. Some non-conventional autoerotic practices can be dangerous. These include erotic asphyxiation and self-bondage. The potential for injury or even death that exists while engaging in the partnered versions of these fetishes (choking and bondage, respectively) becomes drastically increased in the autoerotic case due to the isolation and lack of assistance in the event of a problem.

Sexual activity can be consensual or take place under force or duress, or lawful/illegal or otherwise contrary to social norms or generally accepted sexual morals.

The physiological responses during sexual stimulation are fairly similar for both men and women and there are four phases.[3]

Sexual dysfunction is the inability to react emotionally or physically to sexual stimulation in a way projected of the average healthy person; it can affect different stages in the sexual response cycles, which are desire, excitement and orgasm.[7] In the media, sexual dysfunction is often associated with men, but in actuality, it is more commonly observed in females (43 percent) than males (31 percent).[8]

People engage in sexual activity for any of a multitude of possible reasons. Although the primary evolutionary purpose of sexual activity is reproduction, research on college students suggested that people have sex for four general reasons: physical attraction, as a means to an end, to increase emotional connection, and to alleviate insecurity.[9]

Most people engage in sexual activity because of pleasure they derive from the arousal of their sexuality, especially if they can achieve orgasm. Sexual arousal can also be experienced from foreplay and flirting, and from fetish or BDSM activities,[1][10] or other erotic activities. Most commonly, people engage in sexual activity because of the sexual desire generated by a person to whom they feel sexual attraction; but they may engage in sexual activity for the physical satisfaction they achieve in the absence of attraction for another, as in the case of casual or social sex.[11] At times, a person may engage in a sexual activity solely for the sexual pleasure of their partner, such as because of an obligation they may have to the partner or because of love, sympathy or pity they may feel for the partner.

Also, a person may engage in sexual activity for purely monetary considerations, or to obtain some advantage from either the partner or the activity. A man and woman may engage in sexual intercourse with the objective of conception. Some people engage in hate sex, which occurs between two people who strongly dislike or annoy each other. It is related to the idea that opposition between two people can heighten sexual tension, attraction and interest.[12]

It has been shown that sexual activity plays a large part in the interaction of social species. Joan Roughgarden, in her book Diversity, Gender, and Sexuality in Nature and People, postulates that this applies equally to humans as it does to other social species. She explores the purpose of sexual activity and demonstrates that there are many functions facilitated by such activity including pair bonding, group bonding, dispute resolution and reproduction.[13]

Research has found that people also engage in sexual activity for reasons associated with self-determination theory. The self-determination theory can be applied to a sexual relationship when the participants have positive feelings associated with the relationship. These participants do not feel guilty or coerced into the partnership.[14] Researchers have proposed the model of self-determined sexual motivation. The purpose of this model is to connect self-determination and sexual motivation.[15] This model has helped to explain how people are sexually motivated when involved in self-determined dating relationships. This model also links the positive outcomes, (satisfying the need for autonomy, competence, and relatedness) gained from sexual motivations.[15] According to the completed research associated with this model, it was found that people of both sexes who engaged in sexual activity for self-determined motivation had more positive psychological well-being.[15] While engaging in sexual activity for self-determined reasons, the participants also had a higher need for fulfillment. When this need was satisfied, they felt better about themselves. This was correlated with greater closeness to their partner and higher overall satisfaction in their relationship.[16] Though both sexes engaged in sexual activity for self-determined reasons, there were some differences found between males and females. It was concluded that females had more motivation than males to engage in sexual activity for self-determined reasons.[15] Females also had higher satisfaction and relationship quality than males did from the sexual activity.[15] Overall, research concluded that psychological well-being, sexual motivation, and sexual satisfaction were all positively correlated when dating couples partook in sexual activity for self-determined reasons.[15]

The frequency of sexual activity might range from zero (sexual abstinence) to 15 or 20 times a week.[17] In the United States, the average frequency of sexual intercourse for married couples is 2 to 3 times a week.[18] It is generally recognized that postmenopausal women experience declines in frequency of sexual intercourse[19] and that average frequency of intercourse declines with age. According to the Kinsey Institute, the average frequency of sexual intercourse in the US is 112 times per year (age 1829), 86 times per year (age 3039), and 69 times per year (age 4049).[20]

The age at which adolescents tend to become sexually active varies considerably between different cultures and from time to time. (See Prevalence of virginity.) The first sexual act of a child or adolescent is sometimes referred to as the sexualization of the child, and may be considered as a milestone or a change of status, as the loss of virginity or innocence.

A 1999 survey of students indicated that approximately 40% of ninth graders across the United States report having had sexual intercourse. This figure rises with each grade. Males are more sexually active than females at each of the grade levels surveyed. Sexual activity of young adolescents differs in ethnicity as well. A higher percent of African American and Hispanic adolescents are sexually active than White adolescents.[21]

Research on sexual frequency has also been conducted solely on female adolescents who engage in sexual activity. Female adolescents tended to engage in more sexual activity due to positive mood. In female teenagers, engaging in sexual activity was directly positively correlated with being older, greater sexual activity in the previous week or prior day, and more positive mood the previous day or the same day as the sexual activity occurred.[22] Decreased sexual activity was associated with prior or current day negative mood or vaginal bleeding.[23]

Although opinions differ, others suggest that sexual activity is an essential part of humans, and that teenagers need to experience sex. Sexual experiences help teenagers understand pleasure and satisfaction.[24] In relation to hedonic and eudaimonic well-being, teenagers can positively benefit from sexual activity according to one particular research study. In the United States of America, a cross-sectional study of teenagers was completed. Teenagers who had their first sexual experience at age 16 revealed a higher well-being than those who were sexually inexperienced or who were first sexually active at a later age of 17.[24] Furthermore, teenagers who had their first sexual experience at age 15 or younger, or who had many sexual partners were not negatively affected and did not have associated lower well-being.[24]

Sexual activity is a normal physiological function,[25] but like other physical activity, it comes with risks. There are four main types of risks that may arise from sexual activity: unwanted pregnancy, contracting a sexually transmitted infection (STI/STD), physical injury, and psychological injury.

Any sexual activity that involves the introduction of semen into a woman's vagina, such as during sexual intercourse, or even contact of semen with her vulva, may result in a pregnancy. To reduce the risk of unintended pregnancies, some people who engage in penile-vaginal sex may use contraception, such as birth control pills, a condom, diaphragms, spermicides, hormonal contraception or sterilization.[26] The effectiveness of the various contraceptive methods in avoiding pregnancy varies considerably.

Sexual activity that involves skin-to-skin contact, exposure to an infected person's bodily fluids or mucosal membranes[27] carries the risk of contracting a sexually transmitted infection. People may not be able to detect that their sexual partner has one or more STIs, for example if they are asymptomatic (show no symptoms).[28][29] The risk of STIs can be reduced by safe sex practices, such as using condoms. Both partners may opt be tested for STIs before engaging in sex.[30] There may also be an increased risk of contracting a STI when having sex with multiple partners.

Some STIs can also be contracted by using IV drug needles after their use by an infected person, as well as through childbirth or breastfeeding.

Typically, older men and women maintaining interest in sexual interest and activity could be therapeutic; it is a source of expressing their love and care for one another. Factors such as biological and psychological factors, diseases, mental conditions, boredom with the relationship, and widowhood have been found to contribute with the common decrease in sexual interest and activity in old age. National sex surveys given in Finland in the 1990s revealed aging men, as a result a female widowhood, had a higher incidence of sexual intercourse compared to aging women and that women were more likely to report a lack of sexual desire compared to men. Regression analysis, factors considered important to female sexual activity included: sexual desire, valuing sexuality, and a healthy partner, while high sexual self-esteem, good health, and active sexual history were important to male sexual activity. Both aging genders agreed they needed good health, good sexual functioning, positive sexual self-esteem, and a sexually skilful partner to maintain sexual desire.[31]

Heterosexuality is the romantic or sexual attraction to the opposite sex. Heterosexual sexual practices are subject to laws in many places. In some countries, mostly those where religion has a strong influence on social policy, marriage laws serve the purpose of encouraging people to have sex only within marriage. Sodomy laws were seen as discouraging same-sex sexual practices, but may affect opposite-sex sexual practices. Laws also ban adults from committing sexual abuse, committing sexual acts with anyone under an age of consent, performing sexual activities in public, and engaging in sexual activities for money (prostitution). Though these laws cover both same-sex and opposite-sex sexual activities, they may differ in regard to punishment, and may be more frequently (or exclusively) enforced on those who engage in same-sex sexual activities.[32]

Different-sex sexual practices may be monogamous, serially monogamous, or polyamorous, and, depending on the definition of sexual practice, abstinent or autoerotic (including masturbation). Additionally, different religious and political movements have tried to influence or control changes in sexual practices including courting and marriage, though in most countries changes occur at a slow rate.

Homosexuality is the romantic or sexual attraction to the same sex. People with a homosexual orientation can express their sexuality in a variety of ways, and may or may not express it in their behaviors.[33] Research indicates that many gay men and lesbians want, and succeed in having, committed and durable relationships. For example, survey data indicate that between 40% and 60% of gay men and between 45% and 80% of lesbians are currently involved in a romantic relationship.[34]

It is possible for a person whose sexual identity is mainly heterosexual to engage in sexual acts with people of the same sex. For example, mutual masturbation in the context of what may be considered normal heterosexual teen development. Gay and lesbian people who pretend to be heterosexual are often referred to as being closeted (hiding their sexuality in "the closet"). "Closet case" is a derogatory term used to refer to people who hide their sexuality. Making that orientation public can be called "coming out of the closet" in the case of voluntary disclosure or "outing" in the case of disclosure by others against the subject's wishes (or without their knowledge). Among some communities (called "men on the DL" or "down-low"), same-sex sexual behavior is sometimes viewed as solely for physical pleasure. Men who have sex with men, as well as women who have sex with women, or men on the "down-low" may engage in sex acts with members of the same sex while continuing sexual and romantic relationships with the opposite sex.

People who engage exclusively in same-sex sexual practices may not identify themselves as gay or lesbian. In sex-segregated environments, individuals may seek relationships with others of their own gender (known as situational homosexuality). In other cases, some people may experiment or explore their sexuality with same (and/or different) sex sexual activity before defining their sexual identity. Despite stereotypes and common misconceptions, there are no forms of sexual acts exclusive to same-sex sexual behavior that cannot also be found in opposite-sex sexual behavior, except those involving the meeting of the genitalia between same-sex partners tribadism (generally vulva-to-vulva rubbing, commonly known by its "scissoring" position) and frot (generally penis-to-penis rubbing).

People who have a romantic or sexual attraction to both sexes are referred to as bisexual.[35][36] People who have a distinct but not exclusive preference for one sex/gender over the other may also identify themselves as bisexual.[37] Like gay and lesbian individuals, bisexual people who pretend to be heterosexual are often referred to as being closeted.

Pansexuality (also referred to as omnisexuality)[38] may or may not be subsumed under bisexuality, with some sources stating that bisexuality encompasses sexual or romantic attraction to all gender identities.[39][40] Pansexuality is characterized by the potential for aesthetic attraction, romantic love, or sexual desire towards people without regard for their gender identity or biological sex.[41] Some pansexuals suggest that they are gender-blind; that gender and sex are insignificant or irrelevant in determining whether they will be sexually attracted to others.[42] As defined in the Oxford English Dictionary, pansexuality "encompasses all kinds of sexuality; not limited or inhibited in sexual choice with regards to gender or practice".[43]

Most people experiment with a range of sexual activities during their lives, although they tend to engage in only a few of these regularly. Some people enjoy many different sexual activities, while others avoid sexual activities altogether for religious or other reasons (see chastity, sexual abstinence, asexuality). Some prefer monogamous relationships for sex, while others may prefer many different partners throughout their lives.

Alex Comfort and others propose three potential social aspects of intercourse in humans, which are not mutually exclusive: reproductive, relational, and recreational.[44] The development of the contraceptive pill and other highly effective forms of contraception in the mid- and late 20th century has increased people's ability to segregate these three functions, which still overlap a great deal and in complex patterns. For example: A fertile couple may have intercourse while using contraception to experience sexual pleasure (recreational) and also as a means of emotional intimacy (relational), thus deepening their bonding, making their relationship more stable and more capable of sustaining children in the future (deferred reproductive). This same couple may emphasize different aspects of intercourse on different occasions, being playful during one episode of intercourse (recreational), experiencing deep emotional connection on another occasion (relational), and later, after discontinuing contraception, seeking to achieve pregnancy (reproductive, or more likely reproductive and relational).

Most world religions have sought to address the moral issues that arise from people's sexuality in society and in human interactions. Each major religion has developed moral codes covering issues of sexuality, morality, ethics etc. Though these moral codes do not address issues of sexuality directly, they seek to regulate the situations which can give rise to sexual interest and to influence people's sexual activities and practices. However, the impact of religious teaching has at times been limited. For example, though most religions disapprove of extramarital sexual relations, it has always been widely practiced. Nevertheless, these religious codes have always had a strong influence on peoples' attitudes to issues of modesty in dress, behavior, speech etc.

On the other hand, some people adopt the view that pleasure is its own justification for sexual activity. Hedonism is a school of thought which argues that pleasure is the only intrinsic good.[45]

Human sexual activity, like many other kinds of activity engaged in by humans, is generally influenced by social rules that are culturally specific and vary widely. These social rules are referred to as sexual morality (what can and can not be done by society's rules) and sexual norms (what is and is not expected).

Sexual ethics, morals, and norms relate to issues including deception/honesty, legality, fidelity and consent. Some activities, known as sex crimes in some locations, are illegal in some jurisdictions, including those conducted between (or among) consenting and competent adults (examples include sodomy law and adult-adult incest).

Some people who are in a relationship but want to hide polygamous activity (possibly of opposite sexual orientation) from their partner, may solicit consensual sexual activity with others through personal contacts, online chat rooms, or, advertising in select media.

Swinging, on the other hand, involves singles or partners in a committed relationship engaging in sexual activities with others as a recreational or social activity.[46] The increasing popularity of swinging is regarded by some as arising from the upsurge in sexual activity during the sexual revolution of the 1960s. Swinging sexual activity can take place in a sex club, also known as a swinger club (not to be confused with a strip club).[47]

Some people engage in various sexual activities as a business transaction. When this involves having sex with, or performing certain actual sexual acts for another person in exchange for money or something of value, it is called prostitution. Other aspects of the adult industry include phone sex operators, strip clubs, and pornography.

Social gender roles can influence sexual behavior as well as the reaction of individuals and communities to certain incidents; the World Health Organization states that, "Sexual violence is also more likely to occur where beliefs in male sexual entitlement are strong, where gender roles are more rigid, and in countries experiencing high rates of other types of violence."[48] Some societies, such as those where the concepts of family honor and female chastity are very strong, may practice violent control of female sexuality, through practices such as honor killings and female genital mutilation.[49][50]

The relation between gender equality and sexual expression is recognized, and promotion of equity between men and women is crucial for attaining sexual and reproductive health, as stated by the UN International Conference on Population and Development Program of Action:[51]

B/D, a form of BDSM, is bondage and discipline. Bondage includes the restraint of the body or mind.[52] D/S means "dominant and submissive." A dominant is someone who takes control of someone who wishes to give up control. A submissive is someone who gives up the control to a person who wishes to take control.[52] S/M (sadism and masochism) means an individual who takes pleasure in the humiliation or pain of others. Masochism means an individual who takes pleasure from their own pain and/or humiliation.[52]

In a 2013 study, the researchers suggest that BDSM is a sexual act(s) where they play role games, use restraint, use power exchange, use suppression and pain is sometimes involved depending on individual(s)[53] The study indicates that, in the past, BDSM has been seen as maladaptive to one's psychological health, but that this may be incorrect. According to the study, one who participates in BDSM can have greater strength socially, mentally and have greater independence than those who do not practice BDSM.[53] It states that people who participate in BDSM play actually have higher subjective well-being, and that this might be due to the fact that BDSM play requires extensive communication. Before any sexual act occurs, the partners must discuss their agreement of their relationship. They discuss how long the play will last, the intensity, their actions, what each participant needs and/or desires. The sexual acts are all consensual and pleasurable to both parties.[53]

In a 2015 study, BDSM relationships were suggested to have a higher level of connection, intimacy, trust and communication compared to individuals who do not practice BDSM.[52] The study suggests that dominants and submissives exchange control for each other's pleasure and to satisfy a need. They mention that both parties enjoys pleasing their partner in any way they can. Submissive and Dominants who participated in their research, felt that this is one of the best things about BDSM. It gives a submissive pleasure to do things in general for their dominant. Where Dominant enjoys making their encounters all about the submissive. They enjoy doing things that makes their submissive happy. Their findings suggest that submissives and dominants found BDSM play more pleasurable and fun. BDSM was also suggested to improve personal growth, romantic relationships, their sense of community, their sense of self, the dominants confidence, and help an individual cope with everyday things by giving them a psychological release.[52]

There are many laws and social customs which prohibit, or in some way have an impact on sexual activities. These laws and customs vary from country to country, and have varied over time. They cover, for example, a prohibition to non-consensual sex, to sex outside of marriage, to sexual activity in public, besides many others. Many of these restrictions are non-controversial, but some have been the subject of public debate.

Most societies consider it a serious crime to force someone to engage in sexual acts or to engage in sexual activity with someone who does not consent. This is called sexual assault, and if sexual penetration occurs it is called rape, the most serious kind of sexual assault. The details of this distinction may vary among different legal jurisdictions. Also, what constitutes effective consent in sexual matters varies from culture to culture and is frequently debated. Laws regulating the minimum age at which a person can consent to have sex (age of consent) are frequently the subject of debate, as is adolescent sexual behavior in general. Some societies have forced marriage, where consent may not be required.

Many locales have laws that limit or prohibit same-sex sexual activity.

In the West, sex before marriage is not illegal. There are social taboos and many religions condemn pre-marital sex. In many Muslim countries, such as Saudi Arabia, Pakistan,[54] Afghanistan,[55][56][57] Iran,[57] Kuwait,[58] Maldives,[59] Morocco,[60] Oman,[61] Mauritania,[62] United Arab Emirates,[63][64] Sudan,[65] Yemen,[66] any form of sexual activity outside marriage is illegal. Those found guilty, especially women, may be forced to wed the sexual partner, publicly beaten, or stoned to death.[67] In many African and native tribes, sexual activity is not viewed as a privilege or right of a married couple, but rather as the unification of bodies and is thus not frowned upon.[68]

Other studies have analyzed the changing attitudes about sex that American adolescents have outside of marriage. Adolescents were asked how they felt about oral and vaginal sex in relation to their health, social, and emotional well-being. Overall, teenagers felt that oral sex was viewed as more socially positive amongst their demographic.[69] Results stated that teenagers believed that oral sex for dating and non-dating adolescents was less threatening to their overall values and beliefs than vaginal sex was.[69] When asked, teenagers who participated in the research viewed oral sex as more acceptable to their peers, and their personal values than vaginal sex.[69]

The laws of each jurisdiction set the minimum age at which a young person is allowed to engage in sexual activity.[70] This age of consent is typically between 14 and 18 years, but laws vary. In many jurisdictions, age of consent is a person's mental or functional age.[71][71][72][73] As a result, those above the set age of consent may still be considered unable to legally consent due to mental immaturity.[71][72][73][74][75] Many jurisdictions regard any sexual activity by an adult involving a child as child sexual abuse.

Age of consent may vary by the type of sexual act, the sex of the actors, or other restrictions such as abuse of a position of trust. Some jurisdictions also make allowances for young people engaged in sexual acts with each other.[76]

Most jurisdictions prohibit sexual activity between certain close relatives. These laws vary to some extent; such acts are called incestuous.

Non-consensual sexual activity or subjecting an unwilling person to witnessing a sexual activity are forms of sexual abuse, as well as (in many countries) certain non-consensual paraphilias such as frotteurism, telephone scatophilia (indecent phonecalls), and non-consensual exhibitionism and voyeurism (known as "indecent exposure" and "peeping tom" respectively).[77]

People sometimes exchange sex for money or access to other resources. This practice, called prostitution, takes place under many varied circumstances. The person who receives payment for sexual services is called a prostitute and the person who receives such services is known by a multitude of terms, including (and most commonly) "john." Prostitution is one of the branches of the sex industry. The legal status of prostitution varies from country to country, from being a punishable crime to a regulated profession. Estimates place the annual revenue generated from the global prostitution industry to be over $100 billion.[78] Prostitution is sometimes referred to as "the world's oldest profession".[79] Prostitution may be a voluntary individual activity or facilitated or forced by pimps.

Survival sex is a form of prostitution engaged in by people in extreme need, usually when homeless or otherwise disadvantaged people trade sex for food, a place to sleep, or other basic needs, or for drugs.[80] The term is used by sex trade and poverty researchers and aid workers.[81][82]

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Human sexual activity - Wikipedia, the free encyclopedia

Genetics – Wikipedia, the free encyclopedia

This article is about the general scientific term. For the scientific journal, see Genetics (journal).

Genetics is the study of genes, heredity, and genetic variation in living organisms.[1][2] It is generally considered a field of biology, but it intersects frequently with many of the life sciences and is strongly linked with the study of information systems.

The father of genetics is Gregor Mendel, a late 19th-century scientist and Augustinian friar. Mendel studied 'trait inheritance', patterns in the way traits were handed down from parents to offspring. He observed that organisms (pea plants) inherit traits by way of discrete "units of inheritance". This term, still used today, is a somewhat ambiguous definition of what is referred to as a gene.

Trait inheritance and molecular inheritance mechanisms of genes are still a primary principle of genetics in the 21st century, but modern genetics has expanded beyond inheritance to studying the function and behavior of genes. Gene structure and function, variation, and distribution are studied within the context of the cell, the organism (e.g. dominance) and within the context of a population. Genetics has given rise to a number of sub-fields including epigenetics and population genetics. Organisms studied within the broad field span the domain of life, including bacteria, plants, animals, and humans.

Genetic processes work in combination with an organism's environment and experiences to influence development and behavior, often referred to as nature versus nurture. The intra- or extra-cellular environment of a cell or organism may switch gene transcription on or off. A classic example is two seeds of genetically identical corn, one placed in a temperate climate and one in an arid climate. While the average height of the two corn stalks may be genetically determined to be equal, the one in the arid climate only grows to half the height of the one in the temperate climate, due to lack of water and nutrients in its environment.

The word genetics stems from the Ancient Greek genetikos meaning "genitive"/"generative", which in turn derives from genesis meaning "origin".[3][4][5]

The modern working definition of a gene is a portion (or sequence) of DNA that codes for a known cellular function or process (e.g. the function "make melanin molecules"). A single 'gene' is most similar to a single 'word' in the English language. The nucleotides (molecules) that make up genes can be seen as 'letters' in the English language. Nucleotides are named according to which of the four nitrogenous bases they contain. The four bases are cytosine, guanine, adenine, and thymine. A single gene may have a small number of nucleotides or a large number of nucleotides, in the same way that a word may be small or large (e.g. 'cell' vs. 'electrophysiology'). A single gene often interacts with neighboring genes to produce a cellular function and can even be ineffectual without those neighboring genes. This can be seen in the same way that a 'word' may have meaning only in the context of a 'sentence.' A series of nucleotides can be put together without forming a gene (non coding regions of DNA), like a string of letters can be put together without forming a word (e.g. udkslk). Nonetheless, all words have letters, like all genes must have nucleotides.

A quick heuristic that is often used (but not always true) is "one gene, one protein" meaning a singular gene codes for a singular protein type in a cell (enzyme, transcription factor, etc.)

The sequence of nucleotides in a gene is read and translated by a cell to produce a chain of amino acids which in turn folds into a protein. The order of amino acids in a protein corresponds to the order of nucleotides in the gene. This relationship between nucleotide sequence and amino acid sequence is known as the genetic code. The amino acids in a protein determine how it folds into its unique three-dimensional shape, a structure that is ultimately responsible for the protein's function. Proteins carry out many of the functions needed for cells to live. A change to the DNA in a gene can alter a protein's amino acid sequence, thereby changing its shape and function and rendering the protein ineffective or even malignant (e.g. sickle cell anemia). Changes to genes are called mutations.

The observation that living things inherit traits from their parents has been used since prehistoric times to improve crop plants and animals through selective breeding.[6] The modern science of genetics, seeking to understand this process, began with the work of Gregor Mendel in the mid-19th century.[7]

Although the science of genetics began with the applied and theoretical work of Mendel, other theories of inheritance preceded his work. A popular theory during Mendel's time was the concept of blending inheritance: the idea that individuals inherit a smooth blend of traits from their parents.[8] Mendel's work provided examples where traits were definitely not blended after hybridization, showing that traits are produced by combinations of distinct genes rather than a continuous blend. Blending of traits in the progeny is now explained by the action of multiple genes with quantitative effects. Another theory that had some support at that time was the inheritance of acquired characteristics: the belief that individuals inherit traits strengthened by their parents. This theory (commonly associated with Jean-Baptiste Lamarck) is now known to be wrongthe experiences of individuals do not affect the genes they pass to their children,[9] although evidence in the field of epigenetics has revived some aspects of Lamarck's theory.[10] Other theories included the pangenesis of Charles Darwin (which had both acquired and inherited aspects) and Francis Galton's reformulation of pangenesis as both particulate and inherited.[11]

Modern genetics started with Gregor Johann Mendel, a scientist and Augustinian friar who studied the nature of inheritance in plants. In his paper "Versuche ber Pflanzenhybriden" ("Experiments on Plant Hybridization"), presented in 1865 to the Naturforschender Verein (Society for Research in Nature) in Brnn, Mendel traced the inheritance patterns of certain traits in pea plants and described them mathematically.[12] Although this pattern of inheritance could only be observed for a few traits, Mendel's work suggested that heredity was particulate, not acquired, and that the inheritance patterns of many traits could be explained through simple rules and ratios.

The importance of Mendel's work did not gain wide understanding until the 1890s, after his death, when other scientists working on similar problems re-discovered his research. William Bateson, a proponent of Mendel's work, coined the word genetics in 1905.[13][14] (The adjective genetic, derived from the Greek word genesis, "origin", predates the noun and was first used in a biological sense in 1860.)[15] Bateson both acted as a mentor and was aided significantly by the work of female scientists from Newnham College at Cambridge, specifically the work of Becky Saunders, Nora Darwin Barlow, and Muriel Wheldale Onslow.[16] Bateson popularized the usage of the word genetics to describe the study of inheritance in his inaugural address to the Third International Conference on Plant Hybridization in London, England, in 1906.[17]

After the rediscovery of Mendel's work, scientists tried to determine which molecules in the cell were responsible for inheritance. In 1911, Thomas Hunt Morgan argued that genes are on chromosomes, based on observations of a sex-linked white eye mutation in fruit flies.[18] In 1913, his student Alfred Sturtevant used the phenomenon of genetic linkage to show that genes are arranged linearly on the chromosome.[19]

Although genes were known to exist on chromosomes, chromosomes are composed of both protein and DNA, and scientists did not know which of these is responsible for inheritance. In 1928, Frederick Griffith discovered the phenomenon of transformation (see Griffith's experiment): dead bacteria could transfer genetic material to "transform" other still-living bacteria. Sixteen years later, in 1944, the AveryMacLeodMcCarty experiment identified DNA as the molecule responsible for transformation.[20] The role of the nucleus as the repository of genetic information in eukaryotes had been established by Hmmerling in 1943 in his work on the single celled alga Acetabularia.[21] The HersheyChase experiment in 1952 confirmed that DNA (rather than protein) is the genetic material of the viruses that infect bacteria, providing further evidence that DNA is the molecule responsible for inheritance.[22]

James Watson and Francis Crick determined the structure of DNA in 1953, using the X-ray crystallography work of Rosalind Franklin and Maurice Wilkins that indicated DNA had a helical structure (i.e., shaped like a corkscrew).[23][24] Their double-helix model had two strands of DNA with the nucleotides pointing inward, each matching a complementary nucleotide on the other strand to form what looks like rungs on a twisted ladder.[25] This structure showed that genetic information exists in the sequence of nucleotides on each strand of DNA. The structure also suggested a simple method for replication: if the strands are separated, new partner strands can be reconstructed for each based on the sequence of the old strand. This property is what gives DNA its semi-conservative nature where one strand of new DNA is from an original parent strand.[26]

Although the structure of DNA showed how inheritance works, it was still not known how DNA influences the behavior of cells. In the following years, scientists tried to understand how DNA controls the process of protein production.[27] It was discovered that the cell uses DNA as a template to create matching messenger RNA, molecules with nucleotides very similar to DNA. The nucleotide sequence of a messenger RNA is used to create an amino acid sequence in protein; this translation between nucleotide sequences and amino acid sequences is known as the genetic code.[28]

With the newfound molecular understanding of inheritance came an explosion of research.[29] A notable theory arose from Tomoko Ohta in 1973 with her amendment to the neutral theory of molecular evolution through publishing the nearly neutral theory of molecular evolution. In this theory, Ohta stressed the importance of natural selection and the environment to the rate in which genetic evolution occurs.[30] One important development was chain-termination DNA sequencing in 1977 by Frederick Sanger. This technology allows scientists to read the nucleotide sequence of a DNA molecule.[31] In 1983, Kary Banks Mullis developed the polymerase chain reaction, providing a quick way to isolate and amplify a specific section of DNA from a mixture.[32] The efforts of the Human Genome Project, Department of Energy, NIH, and parallel private effort by Celera Genomics led to the sequencing of the human genome in 2003.[33]

At its most fundamental level, inheritance in organisms occurs by passing discrete heritable units, called genes, from parents to progeny.[34] This property was first observed by Gregor Mendel, who studied the segregation of heritable traits in pea plants.[12][35] In his experiments studying the trait for flower color, Mendel observed that the flowers of each pea plant were either purple or whitebut never an intermediate between the two colors. These different, discrete versions of the same gene are called alleles.

In the case of pea, which is a diploid species, each individual plant has two copies of each gene, one copy inherited from each parent.[36] Many species, including humans, have this pattern of inheritance. Diploid organisms with two copies of the same allele of a given gene are called homozygous at that gene locus, while organisms with two different alleles of a given gene are called heterozygous.

The set of alleles for a given organism is called its genotype, while the observable traits of the organism are called its phenotype. When organisms are heterozygous at a gene, often one allele is called dominant as its qualities dominate the phenotype of the organism, while the other allele is called recessive as its qualities recede and are not observed. Some alleles do not have complete dominance and instead have incomplete dominance by expressing an intermediate phenotype, or codominance by expressing both alleles at once.[37]

When a pair of organisms reproduce sexually, their offspring randomly inherit one of the two alleles from each parent. These observations of discrete inheritance and the segregation of alleles are collectively known as Mendel's first law or the Law of Segregation.

Geneticists use diagrams and symbols to describe inheritance. A gene is represented by one or a few letters. Often a "+" symbol is used to mark the usual, non-mutant allele for a gene.[38]

In fertilization and breeding experiments (and especially when discussing Mendel's laws) the parents are referred to as the "P" generation and the offspring as the "F1" (first filial) generation. When the F1 offspring mate with each other, the offspring are called the "F2" (second filial) generation. One of the common diagrams used to predict the result of cross-breeding is the Punnett square.

When studying human genetic diseases, geneticists often use pedigree charts to represent the inheritance of traits.[39] These charts map the inheritance of a trait in a family tree.

Organisms have thousands of genes, and in sexually reproducing organisms these genes generally assort independently of each other. This means that the inheritance of an allele for yellow or green pea color is unrelated to the inheritance of alleles for white or purple flowers. This phenomenon, known as "Mendel's second law" or the "Law of independent assortment", means that the alleles of different genes get shuffled between parents to form offspring with many different combinations. (Some genes do not assort independently, demonstrating genetic linkage, a topic discussed later in this article.)

Often different genes can interact in a way that influences the same trait. In the Blue-eyed Mary (Omphalodes verna), for example, there exists a gene with alleles that determine the color of flowers: blue or magenta. Another gene, however, controls whether the flowers have color at all or are white. When a plant has two copies of this white allele, its flowers are whiteregardless of whether the first gene has blue or magenta alleles. This interaction between genes is called epistasis, with the second gene epistatic to the first.[40]

Many traits are not discrete features (e.g. purple or white flowers) but are instead continuous features (e.g. human height and skin color). These complex traits are products of many genes.[41] The influence of these genes is mediated, to varying degrees, by the environment an organism has experienced. The degree to which an organism's genes contribute to a complex trait is called heritability.[42] Measurement of the heritability of a trait is relativein a more variable environment, the environment has a bigger influence on the total variation of the trait. For example, human height is a trait with complex causes. It has a heritability of 89% in the United States. In Nigeria, however, where people experience a more variable access to good nutrition and health care, height has a heritability of only 62%.[43]

The molecular basis for genes is deoxyribonucleic acid (DNA). DNA is composed of a chain of nucleotides, of which there are four types: adenine (A), cytosine (C), guanine (G), and thymine (T). Genetic information exists in the sequence of these nucleotides, and genes exist as stretches of sequence along the DNA chain.[44]Viruses are the only exception to this rulesometimes viruses use the very similar molecule RNA instead of DNA as their genetic material.[45] Viruses cannot reproduce without a host and are unaffected by many genetic processes, so tend not to be considered living organisms.

DNA normally exists as a double-stranded molecule, coiled into the shape of a double helix. Each nucleotide in DNA preferentially pairs with its partner nucleotide on the opposite strand: A pairs with T, and C pairs with G. Thus, in its two-stranded form, each strand effectively contains all necessary information, redundant with its partner strand. This structure of DNA is the physical basis for inheritance: DNA replication duplicates the genetic information by splitting the strands and using each strand as a template for synthesis of a new partner strand.[46]

Genes are arranged linearly along long chains of DNA base-pair sequences. In bacteria, each cell usually contains a single circular genophore, while eukaryotic organisms (such as plants and animals) have their DNA arranged in multiple linear chromosomes. These DNA strands are often extremely long; the largest human chromosome, for example, is about 247 million base pairs in length.[47] The DNA of a chromosome is associated with structural proteins that organize, compact and control access to the DNA, forming a material called chromatin; in eukaryotes, chromatin is usually composed of nucleosomes, segments of DNA wound around cores of histone proteins.[48] The full set of hereditary material in an organism (usually the combined DNA sequences of all chromosomes) is called the genome.

While haploid organisms have only one copy of each chromosome, most animals and many plants are diploid, containing two of each chromosome and thus two copies of every gene.[36] The two alleles for a gene are located on identical loci of the two homologous chromosomes, each allele inherited from a different parent.

Many species have so-called sex chromosomes that determine the gender of each organism.[49] In humans and many other animals, the Y chromosome contains the gene that triggers the development of the specifically male characteristics. In evolution, this chromosome has lost most of its content and also most of its genes, while the X chromosome is similar to the other chromosomes and contains many genes. The X and Y chromosomes form a strongly heterogeneous pair.

When cells divide, their full genome is copied and each daughter cell inherits one copy. This process, called mitosis, is the simplest form of reproduction and is the basis for asexual reproduction. Asexual reproduction can also occur in multicellular organisms, producing offspring that inherit their genome from a single parent. Offspring that are genetically identical to their parents are called clones.

Eukaryotic organisms often use sexual reproduction to generate offspring that contain a mixture of genetic material inherited from two different parents. The process of sexual reproduction alternates between forms that contain single copies of the genome (haploid) and double copies (diploid).[36] Haploid cells fuse and combine genetic material to create a diploid cell with paired chromosomes. Diploid organisms form haploids by dividing, without replicating their DNA, to create daughter cells that randomly inherit one of each pair of chromosomes. Most animals and many plants are diploid for most of their lifespan, with the haploid form reduced to single cell gametes such as sperm or eggs.

Although they do not use the haploid/diploid method of sexual reproduction, bacteria have many methods of acquiring new genetic information. Some bacteria can undergo conjugation, transferring a small circular piece of DNA to another bacterium.[50] Bacteria can also take up raw DNA fragments found in the environment and integrate them into their genomes, a phenomenon known as transformation.[51] These processes result in horizontal gene transfer, transmitting fragments of genetic information between organisms that would be otherwise unrelated.

The diploid nature of chromosomes allows for genes on different chromosomes to assort independently or be separated from their homologous pair during sexual reproduction wherein haploid gametes are formed. In this way new combinations of genes can occur in the offspring of a mating pair. Genes on the same chromosome would theoretically never recombine. However, they do via the cellular process of chromosomal crossover. During crossover, chromosomes exchange stretches of DNA, effectively shuffling the gene alleles between the chromosomes.[52] This process of chromosomal crossover generally occurs during meiosis, a series of cell divisions that creates haploid cells.

The first cytological demonstration of crossing over was performed by Harriet Creighton and Barbara McClintock in 1931. Their research and experiments on corn provided cytological evidence for the genetic theory that linked genes on paired chromosomes do in fact exchange places from one homolog to the other.

The probability of chromosomal crossover occurring between two given points on the chromosome is related to the distance between the points. For an arbitrarily long distance, the probability of crossover is high enough that the inheritance of the genes is effectively uncorrelated.[53] For genes that are closer together, however, the lower probability of crossover means that the genes demonstrate genetic linkage; alleles for the two genes tend to be inherited together. The amounts of linkage between a series of genes can be combined to form a linear linkage map that roughly describes the arrangement of the genes along the chromosome.[54]

Genes generally express their functional effect through the production of proteins, which are complex molecules responsible for most functions in the cell. Proteins are made up of one or more polypeptide chains, each of which is composed of a sequence of amino acids, and the DNA sequence of a gene (through an RNA intermediate) is used to produce a specific amino acid sequence. This process begins with the production of an RNA molecule with a sequence matching the gene's DNA sequence, a process called transcription.

This messenger RNA molecule is then used to produce a corresponding amino acid sequence through a process called translation. Each group of three nucleotides in the sequence, called a codon, corresponds either to one of the twenty possible amino acids in a protein or an instruction to end the amino acid sequence; this correspondence is called the genetic code.[55] The flow of information is unidirectional: information is transferred from nucleotide sequences into the amino acid sequence of proteins, but it never transfers from protein back into the sequence of DNAa phenomenon Francis Crick called the central dogma of molecular biology.[56]

The specific sequence of amino acids results in a unique three-dimensional structure for that protein, and the three-dimensional structures of proteins are related to their functions.[57][58] Some are simple structural molecules, like the fibers formed by the protein collagen. Proteins can bind to other proteins and simple molecules, sometimes acting as enzymes by facilitating chemical reactions within the bound molecules (without changing the structure of the protein itself). Protein structure is dynamic; the protein hemoglobin bends into slightly different forms as it facilitates the capture, transport, and release of oxygen molecules within mammalian blood.

A single nucleotide difference within DNA can cause a change in the amino acid sequence of a protein. Because protein structures are the result of their amino acid sequences, some changes can dramatically change the properties of a protein by destabilizing the structure or changing the surface of the protein in a way that changes its interaction with other proteins and molecules. For example, sickle-cell anemia is a human genetic disease that results from a single base difference within the coding region for the -globin section of hemoglobin, causing a single amino acid change that changes hemoglobin's physical properties.[59] Sickle-cell versions of hemoglobin stick to themselves, stacking to form fibers that distort the shape of red blood cells carrying the protein. These sickle-shaped cells no longer flow smoothly through blood vessels, having a tendency to clog or degrade, causing the medical problems associated with this disease.

Some DNA sequences are transcribed into RNA but are not translated into protein productssuch RNA molecules are called non-coding RNA. In some cases, these products fold into structures which are involved in critical cell functions (e.g. ribosomal RNA and transfer RNA). RNA can also have regulatory effect through hybridization interactions with other RNA molecules (e.g. microRNA).

Although genes contain all the information an organism uses to function, the environment plays an important role in determining the ultimate phenotypes an organism displays. This is the complementary relationship often referred to as "nature and nurture". The phenotype of an organism depends on the interaction of genes and the environment. An interesting example is the coat coloration of the Siamese cat. In this case, the body temperature of the cat plays the role of the environment. The cat's genes code for dark hair, thus the hair producing cells in the cat make cellular proteins resulting in dark hair. But these dark hair-producing proteins are sensitive to temperature (i.e. have a mutation causing temperature-sensitivity) and denature in higher-temperature environments, failing to produce dark-hair pigment in areas where the cat has a higher body temperature. In a low-temperature environment, however, the protein's structure is stable and produces dark-hair pigment normally. The protein remains functional in areas of skin that are colder such as its legs, ears, tail and face so the cat has dark-hair at its extremities.[60]

Environment plays a major role in effects of the human genetic disease phenylketonuria.[61] The mutation that causes phenylketonuria disrupts the ability of the body to break down the amino acid phenylalanine, causing a toxic build-up of an intermediate molecule that, in turn, causes severe symptoms of progressive mental retardation and seizures. However, if someone with the phenylketonuria mutation follows a strict diet that avoids this amino acid, they remain normal and healthy.

A popular method in determining how genes and environment ("nature and nurture") contribute to a phenotype is by studying identical and fraternal twins or siblings of multiple births.[62] Because identical siblings come from the same zygote, they are genetically the same. Fraternal siblings are as genetically different from one another as normal siblings. By analyzing statistics on how often a twin of a set has a certain disorder compared to other sets of twins, scientists can determine whether that disorder is caused by genetic or environmental factors (i.e. whether it has 'nature' or 'nurture' causes). One famous example is the multiple birth study of the Genain quadruplets, who were identical quadruplets all diagnosed with schizophrenia.[63]

The genome of a given organism contains thousands of genes, but not all these genes need to be active at any given moment. A gene is expressed when it is being transcribed into mRNA and there exist many cellular methods of controlling the expression of genes such that proteins are produced only when needed by the cell. Transcription factors are regulatory proteins that bind to DNA, either promoting or inhibiting the transcription of a gene.[64] Within the genome of Escherichia coli bacteria, for example, there exists a series of genes necessary for the synthesis of the amino acid tryptophan. However, when tryptophan is already available to the cell, these genes for tryptophan synthesis are no longer needed. The presence of tryptophan directly affects the activity of the genestryptophan molecules bind to the tryptophan repressor (a transcription factor), changing the repressor's structure such that the repressor binds to the genes. The tryptophan repressor blocks the transcription and expression of the genes, thereby creating negative feedback regulation of the tryptophan synthesis process.[65]

Differences in gene expression are especially clear within multicellular organisms, where cells all contain the same genome but have very different structures and behaviors due to the expression of different sets of genes. All the cells in a multicellular organism derive from a single cell, differentiating into variant cell types in response to external and intercellular signals and gradually establishing different patterns of gene expression to create different behaviors. As no single gene is responsible for the development of structures within multicellular organisms, these patterns arise from the complex interactions between many cells.

Within eukaryotes, there exist structural features of chromatin that influence the transcription of genes, often in the form of modifications to DNA and chromatin that are stably inherited by daughter cells.[66] These features are called "epigenetic" because they exist "on top" of the DNA sequence and retain inheritance from one cell generation to the next. Because of epigenetic features, different cell types grown within the same medium can retain very different properties. Although epigenetic features are generally dynamic over the course of development, some, like the phenomenon of paramutation, have multigenerational inheritance and exist as rare exceptions to the general rule of DNA as the basis for inheritance.[67]

During the process of DNA replication, errors occasionally occur in the polymerization of the second strand. These errors, called mutations, can have an impact on the phenotype of an organism, especially if they occur within the protein coding sequence of a gene. Error rates are usually very low1 error in every 10100million basesdue to the "proofreading" ability of DNA polymerases.[68][69] Processes that increase the rate of changes in DNA are called mutagenic: mutagenic chemicals promote errors in DNA replication, often by interfering with the structure of base-pairing, while UV radiation induces mutations by causing damage to the DNA structure.[70] Chemical damage to DNA occurs naturally as well and cells use DNA repair mechanisms to repair mismatches and breaks. The repair does not, however, always restore the original sequence.

In organisms that use chromosomal crossover to exchange DNA and recombine genes, errors in alignment during meiosis can also cause mutations.[71] Errors in crossover are especially likely when similar sequences cause partner chromosomes to adopt a mistaken alignment; this makes some regions in genomes more prone to mutating in this way. These errors create large structural changes in DNA sequence duplications, inversions, deletions of entire regions or the accidental exchange of whole parts of sequences between different chromosomes (chromosomal translocation).

Mutations alter an organism's genotype and occasionally this causes different phenotypes to appear. Most mutations have little effect on an organism's phenotype, health, or reproductive fitness.[72] Mutations that do have an effect are usually deleterious, but occasionally some can be beneficial.[73] Studies in the fly Drosophila melanogaster suggest that if a mutation changes a protein produced by a gene, about 70 percent of these mutations will be harmful with the remainder being either neutral or weakly beneficial.[74]

Population genetics studies the distribution of genetic differences within populations and how these distributions change over time.[75] Changes in the frequency of an allele in a population are mainly influenced by natural selection, where a given allele provides a selective or reproductive advantage to the organism,[76] as well as other factors such as mutation, genetic drift, genetic draft,[77]artificial selection and migration.[78]

Over many generations, the genomes of organisms can change significantly, resulting in evolution. In the process called adaptation, selection for beneficial mutations can cause a species to evolve into forms better able to survive in their environment.[79] New species are formed through the process of speciation, often caused by geographical separations that prevent populations from exchanging genes with each other.[80] The application of genetic principles to the study of population biology and evolution is known as the "modern synthesis".

By comparing the homology between different species' genomes, it is possible to calculate the evolutionary distance between them and when they may have diverged. Genetic comparisons are generally considered a more accurate method of characterizing the relatedness between species than the comparison of phenotypic characteristics. The evolutionary distances between species can be used to form evolutionary trees; these trees represent the common descent and divergence of species over time, although they do not show the transfer of genetic material between unrelated species (known as horizontal gene transfer and most common in bacteria).[81]

Although geneticists originally studied inheritance in a wide range of organisms, researchers began to specialize in studying the genetics of a particular subset of organisms. The fact that significant research already existed for a given organism would encourage new researchers to choose it for further study, and so eventually a few model organisms became the basis for most genetics research.[82] Common research topics in model organism genetics include the study of gene regulation and the involvement of genes in development and cancer.

Organisms were chosen, in part, for convenienceshort generation times and easy genetic manipulation made some organisms popular genetics research tools. Widely used model organisms include the gut bacterium Escherichia coli, the plant Arabidopsis thaliana, baker's yeast (Saccharomyces cerevisiae), the nematode Caenorhabditis elegans, the common fruit fly (Drosophila melanogaster), and the common house mouse (Mus musculus).

Medical genetics seeks to understand how genetic variation relates to human health and disease.[83] When searching for an unknown gene that may be involved in a disease, researchers commonly use genetic linkage and genetic pedigree charts to find the location on the genome associated with the disease. At the population level, researchers take advantage of Mendelian randomization to look for locations in the genome that are associated with diseases, a method especially useful for multigenic traits not clearly defined by a single gene.[84] Once a candidate gene is found, further research is often done on the corresponding gene the orthologous gene in model organisms. In addition to studying genetic diseases, the increased availability of genotyping methods has led to the field of pharmacogenetics: the study of how genotype can affect drug responses.[85]

Individuals differ in their inherited tendency to develop cancer,[86] and cancer is a genetic disease.[87] The process of cancer development in the body is a combination of events. Mutations occasionally occur within cells in the body as they divide. Although these mutations will not be inherited by any offspring, they can affect the behavior of cells, sometimes causing them to grow and divide more frequently. There are biological mechanisms that attempt to stop this process; signals are given to inappropriately dividing cells that should trigger cell death, but sometimes additional mutations occur that cause cells to ignore these messages. An internal process of natural selection occurs within the body and eventually mutations accumulate within cells to promote their own growth, creating a cancerous tumor that grows and invades various tissues of the body.

Normally, a cell divides only in response to signals called growth factors and stops growing once in contact with surrounding cells and in response to growth-inhibitory signals. It usually then divides a limited number of times and dies, staying within the epithelium where it is unable to migrate to other organs. To become a cancer cell, a cell has to accumulate mutations in a number of genes (37) that allow it to bypass this regulation: it no longer needs growth factors to divide, it continues growing when making contact to neighbor cells, and ignores inhibitory signals, it will keep growing indefinitely and is immortal, it will escape from the epithelium and ultimately may be able to escape from the primary tumor, cross the endothelium of a blood vessel, be transported by the bloodstream and will colonize a new organ, forming deadly metastasis. Although there are some genetic predispositions in a small fraction of cancers, the major fraction is due to a set of new genetic mutations that originally appear and accumulate in one or a small number of cells that will divide to form the tumor and are not transmitted to the progeny (somatic mutations). The most frequent mutations are a loss of function of p53 protein, a tumor suppressor, or in the p53 pathway, and gain of function mutations in the ras proteins, or in other oncogenes.

DNA can be manipulated in the laboratory. Restriction enzymes are commonly used enzymes that cut DNA at specific sequences, producing predictable fragments of DNA.[88] DNA fragments can be visualized through use of gel electrophoresis, which separates fragments according to their length.

The use of ligation enzymes allows DNA fragments to be connected. By binding ("ligating") fragments of DNA together from different sources, researchers can create recombinant DNA, the DNA often associated with genetically modified organisms. Recombinant DNA is commonly used in the context of plasmids: short circular DNA molecules with a few genes on them. In the process known as molecular cloning, researchers can amplify the DNA fragments by inserting plasmids into bacteria and then culturing them on plates of agar (to isolate clones of bacteria cells). ("Cloning" can also refer to the various means of creating cloned ("clonal") organisms.)

DNA can also be amplified using a procedure called the polymerase chain reaction (PCR).[89] By using specific short sequences of DNA, PCR can isolate and exponentially amplify a targeted region of DNA. Because it can amplify from extremely small amounts of DNA, PCR is also often used to detect the presence of specific DNA sequences.

DNA sequencing, one of the most fundamental technologies developed to study genetics, allows researchers to determine the sequence of nucleotides in DNA fragments. The technique of chain-termination sequencing, developed in 1977 by a team led by Frederick Sanger, is still routinely used to sequence DNA fragments.[90] Using this technology, researchers have been able to study the molecular sequences associated with many human diseases.

As sequencing has become less expensive, researchers have sequenced the genomes of many organisms, using a process called genome assembly, which utilizes computational tools to stitch together sequences from many different fragments.[91] These technologies were used to sequence the human genome in the Human Genome Project completed in 2003.[33] New high-throughput sequencing technologies are dramatically lowering the cost of DNA sequencing, with many researchers hoping to bring the cost of resequencing a human genome down to a thousand dollars.[92]

Next generation sequencing (or high-throughput sequencing) came about due to the ever-increasing demand for low-cost sequencing. These sequencing technologies allow the production of potentially millions of sequences concurrently.[93][94] The large amount of sequence data available has created the field of genomics, research that uses computational tools to search for and analyze patterns in the full genomes of organisms. Genomics can also be considered a subfield of bioinformatics, which uses computational approaches to analyze large sets of biological data. A common problem to these fields of research is how to manage and share data that deals with human subject and personally identifiable information. See also genomics data sharing.

On 19 March 2015, scientists urged a worldwide ban on clinical use of methods, particularly the use of CRISPR and zinc finger, to edit the human genome in a way that can be inherited.[95][96][97][98] In April 2015, Chinese researchers reported results of basic research to edit the DNA of non-viable human embryos using CRISPR.[99][100]

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