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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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Genetics - Wikipedia, the free encyclopedia

Genetics | The Biology Corner

Genetics includes the study of heredity, or how traits are passed from parents to offspring. The topics of genetics vary and are constantly changing as we learn more about the genome and how we are influenced by our genes.

Mendel & Inheritance powerpoint presentation covering basics of genetics

Heredity Simulation use popsicle sticks to show how alleles are inherited Penny Genetics flip a coin to compare actual outcomes versus predicted outcomes from a punnett square Heredity Wordsearch fill in the blank, find words

Simple Genetics Practice using mendelian genetics and punnett squares

Genetic Crosses with two traits basic crosses, uses Punnet squares Genetic Crosses with two traits II basic crossses, uses Punnett squares Dihybrid Crosses in Guinea Pigs(pdf) step through on how to do a 44 punnett square

Codominance & Incomplete Dominance basic crosses involving codominance

Genetics Practice Problems includes codominance, multiple allele traits, polygenic traits, for AP Biology Genetics Practice Problems II for advanced biology students, includes both single allele and dihybrid crosses, intended for practice after students have learned multiplicative properties of statistics and mathematical analysis of genetic crosses

X-Linked Traits practice crosses that involve sex-linkage, mainly in fruitflies

X Linked Genetics in Calico Cats more practice with sex-linked traits Multiple Allele Traits practice with blood type crosses and other ABO type alleles Multiple Allele Traits in Chickens shows how combs are inherited (rrpp x RRpp) Inheritance and Eye Color uses a simulation to show how multiple alleles can influence a single trait (eye color)

The Genetics of Blood Disorders a worksheet with genetics problems that relate to specific disorders: sickle cell anemia, hemophilia, and Von Willebrand disease.

Oompa Loompa Genetics(pdf) basic crosses and problem sets, using oompa loompas Norn Genetics online simulation showing basic single allele traits, multiple allele traits and codominance

Human Genetics Survey class takes a survey of human traits, such as ear points Human Genetics Bingo grid with traits, powerpoint presentation discusses traits Human Genetics Presentation discusses ABO blood types, albinism, cystic fibrosis and other traits unique to humans

Study Guides from Biology101.org

Design-a-Species using the rules of inheritance (mendel), create an organism; dominance & recessiveness, multiple allele traits, codominance Variations on a Human Face toss a penny to determine the features of a face, such as freckles, dimples; then draw that face. Paper Pets another simulation using paper models with traits for eyes, nose, mouth, and hair.

Hardy-Weinberg Problem Set statistical analysis, using HW equation and some dragons Hardy Weinberg Simulation track an allele in population by simulating how parents pass alleles to offspring

Corn Genetics and Chi Square statistical analysis, using preserved corn and counting kernals Corn Genetics grow corn, 3:1 albino ratio, lab report analyzes F1, F2 crosses

Fruit Fly Genetics virtual lab where you cross different flies, gather data and statistically analyze the results Fruit Fly (Drosophila) Virtual Lab more extensive virtual lab through a program created by Virtual Courseware, requires set up by teacher. Drosophilab this virtual lab requires you to download a program to your computer, students can choose traits to cross and run chi square analysis on outcomes, while this is more basic than the Virtual Courseware lab, it appears to have less bugs.

Dragon Genetics Word Problems (ppt) displays genetics problems on projector for students to solve

Meiosis Label look at cells in various stages of meiosis, identify and order Meiosis Internet Lesson look at animations of meiosis and answer questions Meiosis Powerpoint slideshow covers meiosis, homologous chromosomes, crossing over

Modeling Chromosomal Inheritance use pipe cleaners to show how genes are inherited; independent assortment, segregation, sex-linkage

Linkage Group Simulation uses pipe cleaners and beads, students construct chromosomes with alleles and perform crosses, predicting outcomes (advanced) Karyotyping Online use a website simulator to learn how to pair chromosomes and diagnose abnormalities Karyotyping Online II another simulation on how to construct a karyotype Chromosome Study cut out chromosomes and tape them in pairs to construct a paper karyotype

Gender and Sex Determination NOVA explores how sex is determined, and social issues of gender

DNA Powerpoint Presentation covers the basics for a freshman level class

DNA Coloring basic image of DNA and RNA DNA Crossword basic terms Transcription & Translation Coloring shows structures involved, nucleotides, base pair rules, amino acids DNA Analysis: Recombination simulate DNA recombination using paper slips and sequences DNA Extraction instructions for extracting DNA from a strawberry, very simple, works every time! DNA in Snorks analyze and transcribe DNA sequences, construct a creature based on that sequence

How DNA Controls the Workings of a Cell examine a DNA sequence, transcribe and translate DNA Sequencing in Bacteria website simulates the sequencing of bacterial DNA, PCR techniques Ramalian DNA imagine an alien species that has triple-stranded DNA, base pair rules still apply Who Ate The Cheese simulate gel electrophoresis to solve a crime HIV Coloring shows how viral DNA enters and infects a cell

Genetic Science Ethics survey as a group ethical questions involved genetics (cloning, gene therapy..) Your Genes Your Choices this is a more involved group assignment where groups read scenarios about genetic testing and ethics involved. Genetic Engineering Concept Map Complete this graphic organizer on various techniques used in genetics, such as selective breeding and manipulating DNA

Genetic Engineering presentation on cloning, recombinant DNA, and gel electrophoresis Biotechnology Web Lesson students explore genetic science learning center (http://learn.genetics.utah.edu/) and discover how clones are made, and how DNA is extracted and sequenced Genetic Science Learning Center explore website with animations and tutorials, answer questions

DNA From the Beginning -step by step tutorial on the discovery of genes, DNA, and how they control traits, site by Dolan DNA Learning Center DNA Fingerprinting another simulation, this one from PBS, that walks you through the steps of creating a DNA Fingerprint Cloning Click and Clone at GSLC where you can read about how clones are made and clone your own virtual mouse

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Genetics | The Biology Corner

Learn Genetics

Tour of Basic Genetics

Characteristics of Inheritance

Molecules of Inheritance

Pigeon Breeding: Genetics at Work

Chromosomes & Inheritance

Genetic Variation

Variation + Selection & Time

Epigenetics

Genetic Science

Genetic Disorders

Pharmacogenomics

Family Health History

Gene Therapy

Amazing Cells

Stem Cells

Cloning

The Human Microbiome

Model Earth

Astrobiology

Extreme Environments: Great Salt Lake

Addiction: Genetics & the Brain

Sensory Systems

APA format: Genetic Science Learning Center (2014, June 13) Learn Genetics. Learn.Genetics. Retrieved October 20, 2015, from http://learn.genetics.utah.edu/ MLA format: Genetic Science Learning Center. "Learn Genetics." Learn.Genetics 20 October 2015 <http://learn.genetics.utah.edu/> Chicago format: Genetic Science Learning Center, "Learn Genetics," Learn.Genetics, 13 June 2014, <http://learn.genetics.utah.edu/> (20 October 2015)

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Learn Genetics

What is Genetics? – News Medical

By Dr Ananya Mandal, MD

Genetics is the study of heredity. Heredity is a biological process where a parent passes certain genes onto their children or offspring. Every child inherits genes from both of their biological parents and these genes in turn express specific traits. Some of these traits may be physical for example hair and eye color and skin color etc. On the other hand some genes may also carry the risk of certain diseases and disorders that may pass on from parents to their offspring.

The genetic information lies within the cell nucleus of each living cell in the body. The information can be considered to be retained in a book for example. Part of this book with the genetic information comes from the father while the other part comes from the mother.

The genes lie within the chromosomes. Humans have 23 pairs of these small thread-like structures in the nucleus of their cells. 23 or half of the total 46 comes from the mother while the other 23 comes from the father.

The chromosomes contain genes just like pages of a book. Some chromosomes may carry thousands of important genes while some may carry only a few. The chromosomes, and therefore the genes, are made up of the chemical substance called DNA (DeoxyriboNucleic Acid). The chromosomes are very long thin strands of DNA, coiled up tightly.

At one point along their length, each chromosome has a constriction, called the centromere. The centromere divides the chromosomes into two arms: a long arm and a short arm. Chromosomes are numbered from 1 to 22 and these are common for both sexes and called autosomes. There are also two chromosomes that have been given the letters X and Y and termed sex chromosomes. The X chromosome is much larger than the Y chromosome.

The genes are further made up of unique codes of chemical bases comprising of A, T, C and G (Adenine, Thymine, Cytosine and Guanine). These chemical bases make up combinations with permutations and combinations. These are akin to the words on a page.

These chemical bases are part of the DNA. The words when stringed together act as the blueprints that tells the cells of the body when and how to grow, mature and perform various functions. With age the genes may be affected and may develop faults and damages due to environmental and endogenous toxins.

Women have 46 chromosomes (44 autosomes plus two copies of the X chromosome) in their body cells. They have half of this or 22 autosomes plus an X chromosome in their egg cells.

Men have 46 chromosomes (44 autosomes plus an X and a Y chromosome) in their body cells and have half of these 22 autosomes plus an X or Y chromosome in their sperm cells.

When the egg joins with the sperm, the resultant baby has 46 chromosomes (with either an XX in a female baby or XY in a male baby).

Each gene is a piece of genetic information. All the DNA in the cell makes up for the human genome. There are about 20,000 genes located on one of the 23 chromosome pairs found in the nucleus.

To date, about 12,800 genes have been mapped to specific locations (loci) on each of the chromosomes. This database was begun as part of the Human Genome Project. The project was officially completed in April 2003 but the exact number of genes in the human genome is still unknown.

Reviewed by April Cashin-Garbutt, BA Hons (Cantab)

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What is Genetics? - News Medical

Genetics – definition of genetics by The Free Dictionary

genetics - the branch of biology that studies heredity and variation in organisms genetic science transformation - (genetics) modification of a cell or bacterium by the uptake and incorporation of exogenous DNA hybridisation, hybridization, hybridizing, interbreeding, crossbreeding, crossing, cross - (genetics) the act of mixing different species or varieties of animals or plants and thus to produce hybrids chromosome mapping, mapping - (genetics) the process of locating genes on a chromosome carrier - (genetics) an organism that possesses a recessive gene whose effect is masked by a dominant allele; the associated trait is not apparent but can be passed on to offspring amphidiploid - (genetics) an organism or cell having a diploid set of chromosomes from each parent diploid - (genetics) an organism or cell having the normal amount of DNA per cell; i.e., two sets of chromosomes or twice the haploid number haploid - (genetics) an organism or cell having only one complete set of chromosomes heteroploid - (genetics) an organism or cell having a chromosome number that is not an even multiple of the haploid chromosome number for that species polyploid - (genetics) an organism or cell having more than twice the haploid number of chromosomes crossbreed, hybrid, cross - (genetics) an organism that is the offspring of genetically dissimilar parents or stock; especially offspring produced by breeding plants or animals of different varieties or breeds or species; "a mule is a cross between a horse and a donkey" vector - (genetics) a virus or other agent that is used to deliver DNA to a cell cosmid - (genetics) a large vector that is made from a bacteriophage and used to clone genes or gene fragments character - (genetics) an attribute (structural or functional) that is determined by a gene or group of genes unit character - (genetics) a character inherited on an all-or-none basis and dependent on the presence of a single gene hereditary pattern, inheritance - (genetics) attributes acquired via biological heredity from the parents heterosis, hybrid vigor - (genetics) the tendency of a crossbred organism to have qualities superior to those of either parent gene linkage, linkage - (genetics) traits that tend to be inherited together as a consequence of an association between their genes; all of the genes of a given chromosome are linked (where one goes they all go) fertilized ovum, zygote - (genetics) the diploid cell resulting from the union of a haploid spermatozoon and ovum (including the organism that develops from that cell) heterozygote - (genetics) an organism having two different alleles of a particular gene and so giving rise to varying offspring homozygote - (genetics) an organism having two identical alleles of a particular gene and so breeding true for the particular characteristic cistron, gene, factor - (genetics) a segment of DNA that is involved in producing a polypeptide chain; it can include regions preceding and following the coding DNA as well as introns between the exons; it is considered a unit of heredity; "genes were formerly called factors" allele, allelomorph - (genetics) either of a pair (or series) of alternative forms of a gene that can occupy the same locus on a particular chromosome and that control the same character; "some alleles are dominant over others" haplotype - (genetics) a combination of alleles (for different genes) that are located closely together on the same chromosome and that tend to be inherited together XX - (genetics) normal complement of sex chromosomes in a female XXX - (genetics) abnormal complement of three X chromosomes in a female XXY - (genetics) abnormal complement of sex hormones in a male resulting in Klinefelter's syndrome XY - (genetics) normal complement of sex hormones in a male XYY - (genetics) abnormal complement of sex hormones in a male who has two Y chromosomes sex chromosome - (genetics) a chromosome that determines the sex of an individual; "mammals normally have two sex chromosomes" Mendel's law - (genetics) one of two principles of heredity formulated by Gregor Mendel on the basis of his experiments with plants; the principles were limited and modified by subsequent genetic research biological science, biology - the science that studies living organisms cytogenetics - the branch of biology that studies the cellular aspects of heredity (especially the chromosomes) genomics - the branch of genetics that studies organisms in terms of their genomes (their full DNA sequences) proteomics - the branch of genetics that studies the full set of proteins encoded by a genome molecular genetics - the branch of genetics concerned with the structure and activity of genetic material at the molecular level pharmacogenetics - the branch of genetics that studies the genetically determined variations in responses to drugs in humans or laboratory organisms recombination - (genetics) a combining of genes or characters different from what they were in the parents chromosomal mutation, genetic mutation, mutation - (genetics) any event that changes genetic structure; any alteration in the inherited nucleic acid sequence of the genotype of an organism

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Genetics - definition of genetics by The Free Dictionary

Genetics Home Reference – Your guide to understanding genetic …

The genetics of more than 1,000 health conditions, diseases, and syndromes.

More than 1,300 genes, health effects of genetic differences, and gene families.

Chromosomes, mitochondrial DNA, and associated health conditions.

Learn about mutations, inheritance, genetic counseling, genetic testing, genomic research, and more.

Medical and genetics definitions.

Links to other genetics information and organizations.

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Genetics Home Reference - Your guide to understanding genetic ...

What is Embryology?

By Dr Ananya Mandal, MD

Embryology is the study of development of an embryo from the stage of ovum fertilization through to the fetal stage.

The ball of dividing cells that results after fertilization is termed an embryo for eight weeks and from nine weeks after fertilization, the term used is fetus.

Once an egg is released from the ovary during ovulation, it meets with a sperm cell that was carried to it via the semen. These two gametes combine to form a zygote and this process is called fertilization. The zygote then begins to divide and becomes a blastula.

The blastula develops in one of two ways, which actually divides the whole animal kingdom in half. The blastula develops a pore at one end, called a blastopore. If that blastopore becomes the mouth of the animal, the animal is a protostome, and if it forms an anus, the animal is a deuterostome.

Protosomes are invertebrate animals such as worms, insects and molluscs while deuterostomes are vertebrates such as birds, reptiles, and humans.

The blastula continues to develop, eventually forming a structure called the gastrula. The gastrula then forms three germ cell layers, from which all of the bodys organs and tissues are eventually derived. From the innermost layer or endoderm, the digestive organs, lungs and bladder develop; the skeleton, blood vessels and muscles are derived from the middle layer or mesoderm and the outer layer or ectoderm gives rise to the nervous system, skin and hair.

Reviewed by Sally Robertson, BSc

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What is Embryology?

Embryology | Define Embryology at Dictionary.com

British Dictionary definitions for embryology Expand

the branch of science concerned with the study of embryos

the structure and development of the embryo of a particular organism

Derived Forms

embryological (mbrldkl), embryologic, adjectiveembryologically, adverbembryologist, noun

Word Origin and History for embryology Expand

1859, from embryon (see embryo) + -logy. Related: Embryologist (c.1850).

embryology in Medicine Expand

embryology embryology (m'br-l'-j) n.

The branch of biology that deals with the formation, early growth, and development of living organisms.

The embryonic structure or development of an organism.

embryology in Science Expand

embryology in Culture Expand

The study of the embryo; a major field of research in modern biology.

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Embryology | Define Embryology at Dictionary.com

Embryology – Wikipedia, the free encyclopedia

This article is about the development of embryos in animals. For the development of plant embryos, see Sporophyte.

Embryology (from Greek , embryon, "the unborn, embryo"; and -, -logia) is the branch of biology that studies the development of gametes (sex cells), fertilization, and development of embryos and fetuses. Additionally, embryology is the study of congenital disorders that occur before birth.[1]

After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.

In bilateral animals, the blastula develops in one of two ways that divides the whole animal kingdom into two halves (see: Embryological origins of the mouth and anus). If in the blastula the first pore (blastopore) becomes the mouth of the animal, it is a protostome; if the first pore becomes the anus then it is a deuterostome. The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes include the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula.

The gastrula with its blastopore soon develops three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop:

Embryos in many species often appear similar to one another in early developmental stages. The reason for this similarity is because species have a shared evolutionary history. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor.

Click here to read the main article on Drosophila embryogenesis

Drosophila melanogaster, a fruit fly, is a model organism in biology on which much research into embryology has been done (see figure 1.1.1A and figure 1.1.1B).[2] Before fertilization, the female gamete produces an abundance of mRNA - transcribed from the genes that encode bicoid protein and nanos protein.[3][4] These mRNA molecules are stored to be used later in what will become a developing embryo. The male and female Drosophila gametes exhibit anisogamy (differences in morphology and sub-cellular biochemistry). The female gamete is larger than the male gamete because it harbors more cytoplasm and, within the cytoplasm, the female gamete contains an abundance of the mRNA previously mentioned.[5][6] At fertilization, the male and female gametes fuse (plasmogamy) and then the nucleus of the male gamete fuses with the nucleus of the female gamete (karyogamy). Note that before the gametes' nuclei fuse, they are known as pronuclei.[7] A series of nuclear divisions will occur without cytokinesis (division of the cell) in the zygote to form a multi-nucleated cell (a cell containing multiple nuclei) known as a syncytium.[8][9] All the nuclei in the syncytium are identical, just as all the nuclei in every somatic cell of any multicellular organism are identical in terms of the DNA sequence of the genome.[10] Before the nuclei can differentiate in transcriptional activity, the embryo (syncytium) must be divided into segments. In each segment, a unique set of regulatory proteins will cause specific genes in the nuclei to be transcribed. The resulting combination of proteins will transform clusters of cells into early embryo tissues that will each develop into multiple fetal and adult tissues later in development (note: this happens after each nucleus becomes wrapped with its own cell membrane).

Outlined below is the process that leads to cell and tissue differentiation.

Maternal-effect genes - subject to Maternal (cytoplasmic) inheritance.

Zygotic-effect genes - subject to Mendelian (classical) inheritance.

Humans are bilaterals and deuterostomes.

In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception (tenth week of pregnancy), the developing human is then called a fetus.

As recently as the 18th century, the prevailing notion in western human embryology was preformation: the idea that semen contains an embryo a preformed, miniature infant, or homunculus that simply becomes larger during development. The competing explanation of embryonic development was epigenesis, originally proposed 2,000 years earlier by Aristotle. Much early embryology came from the work of the Italian anatomists Aldrovandi, Aranzio, Leonardo da Vinci, Marcello Malpighi, Gabriele Falloppio, Girolamo Cardano, Emilio Parisano, Fortunio Liceti, Stefano Lorenzini, Spallanzani, Enrico Sertoli, and Mauro Rusconi.[22] According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favoured explanation among embryologists.[23]

Karl Ernst von Baer and Heinz Christian Pander proposed the germ layer theory of development; von Baer discovered the mammalian ovum in 1827.[24][25][26] Modern embryological pioneers include Charles Darwin, Ernst Haeckel, J.B.S. Haldane, and Joseph Needham. Other important contributors include William Harvey, Kaspar Friedrich Wolff, Heinz Christian Pander, August Weismann, Gavin de Beer, Ernest Everett Just, and Edward B. Lewis.

After the 1950s, with the DNA helical structure being unravelled and the increasing knowledge in the field of molecular biology, developmental biology emerged as a field of study which attempts to correlate the genes with morphological change, and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulated.

Many principles of embryology apply to invertebrates as well as to vertebrates.[27] Therefore, the study of invertebrate embryology has advanced the study of vertebrate embryology. However, there are many differences as well. For example, numerous invertebrate species release a larva before development is complete; at the end of the larval period, an animal for the first time comes to resemble an adult similar to its parent or parents. Although invertebrate embryology is similar in some ways for different invertebrate animals, there are also countless variations. For instance, while spiders proceed directly from egg to adult form, many insects develop through at least one larval stage.

Currently, embryology has become an important research area for studying the genetic control of the development process (e.g. morphogens), its link to cell signalling, its importance for the study of certain diseases and mutations, and in links to stem cell research.

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Embryology - Wikipedia, the free encyclopedia

Embryology | definition of embryology by Medical dictionary

embryology [embre-olo-je]

the science of the development of the individual during the embryonic stage and, by extension, in several or even all preceding and subsequent stages of the life cycle. adj., adj embryologic.

Science of the origin and development of the organism from fertilization of the oocyte to the end of the eighth week. Usually used to include all stages of prenatal life.

[embryo- + G. logos, study]

1. The branch of biology that deals with the formation, early growth, and development of living organisms.

2. The embryonic structure or development of an organism.

Etymology: Gk, en, bryein + logos, science

Science of the origin and development of the organism from fertilization of the oocyte to the end of the eighth week and, by extension, all subsequent stages up to birth.

[embryo- + G. logos, study]

Science of the origin and development of the organism from fertilization of the oocyte to the end of the eighth week.

[embryo- + G. logos, study]

(embrolj), n the study of the origin, growth, development, and function of an organism from fertilization to birth.

the science of the development of the individual animal during the embryonic stage and, by extension, in several or even all preceding and subsequent stages of the life cycle.

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Embryology | definition of embryology by Medical dictionary