scientific study of the mechanism of heredity. While Gregor
first presented his findings on the statistical laws governing the transmission of certain traits from generation to generation in 1856, it was not until the discovery and detailed study of the
in the 20th cent. that scientists found the physical basis of hereditary characteristics. A brief summary of the basic laws of heredity and the terms used follows.
The gene is defined as the unit of inheritance. A gene is actually a sequence of DNA (see nucleic acidnucleic acid, any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis. ..... Click the link for more information. ) contained by and arranged linearly along a chromosome. Each gene transmits chemical information that is expressed as a trait, e.g., tall or dwarf size in the garden pea plant. Each species has a
genome, or characteristic set of genes, that contains the total genetic information for an individual organism. In many familiar organisms two genes for each trait are present in each individual, and these paired genes, both governing the same trait, are called
alleles. The two allelic genes in any one individual may be alike (homozygous) or different (heterozygous). The chromosomes of animals and plants that reproduce sexually usually exist in pairs; the members of a chromosome pair are termed homologous (see reproductionreproduction, capacity of all living systems to give rise to new systems similar to themselves. The term reproduction may refer to this power of self-duplication of a single cell or a multicellular animal or plant organism. ..... Click the link for more information. ). In humans there are 46 chromosomes, or 23 homologous pairs. Pairs of genes are borne on homologous chromosomes.
In the process of meiosismeiosis , process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed. ..... Click the link for more information. , by which ova and sperm are produced, the chromosomes are so divided that each mature sex cell contains half the original number of chromosomes, or one chromosome of each pair, and therefore one gene of each pair. Thus, when the ovum and the sperm fuse on fertilization, the fertilized egg (zygote) receives one allele from each parent. With many pairs of alleles that have contrasting effects (e.g., certain alleles produce different eye color), one is dominant and the other recessive: an individual heterozygous (carrying contrasting alleles) for a given characteristic invariably displays one aspect of that characteristic and not its alternative, although the gene for the aspect that does not appear (i.e., that is recessive) is present. This individual is called a hybridhybrid , term applied by plant and animal breeders to the offspring of a cross between two different subspecies or species, and by geneticists to the offspring of parents differing in any genetic characteristic (see genetics). ..... Click the link for more information. .
In Mendelian law (see MendelMendel, Gregor Johann , 182284, Austrian monk noted for his experimental work on heredity. He entered the Augustinian monastery in Brno in 1843, taught at a local secondary school, and carried out independent scientific investigations on garden peas and other plants until ..... Click the link for more information. ) the offspringor first filial (called F1) generationof parents each homozygous for different alleles of a given gene are all hybrids heterozygous for the characteristic governed by that gene and are said to be of the same
phenotype, i.e., they are all similar in appearance to the homozygous dominant parent because the recessive characteristic is masked, although their gene composition, or
genotype, is different from either parent. A cross of members of the F1 generation produces a second filial (F2) generation of which approximately three fourths show the dominant characteristic and one fourth the recessive. Note however, that great numbers of characteristics are inherited simultaneously and the patterns of transmission of genes are such that offspring strongly resembling one parent in certain traits can resemble the other parent in other traits.
It has also become clear that an individual organism's heredity and environment interact in the manifestation of many traits: a pea plant with a genetic tendency toward tallness will not achieve its full size if deprived of adequate water and minerals for growth. However, true alterations in gene and chromosome structure are the product of mutationmutation, in biology, a sudden, random change in a gene, or unit of hereditary material, that can alter an inheritable characteristic. Most mutations are not beneficial, since any change in the delicate balance of an organism having a high level of adaptation to its environment ..... Click the link for more information. and are not produced by environmental conditions, as was postulated by the theory of acquired characteristicsacquired characteristics, modifications produced in an individual plant or animal as a result of mutilation, disease, use and disuse, or any distinctly environmental influence. Some examples are docking of tails, malformation caused by disease, and muscle atrophy. ..... Click the link for more information. . The discovery by H. J. MullerMuller, Hermann Joseph , 18901967, American geneticist and educator, b. New York City, grad. Columbia (B.A., 1910; Ph.D., 1916). A student of Thomas Hunt Morgan, he taught (191518) at Rice Institute, Tex., at Columbia (191820), and at the Univ. ..... Click the link for more information. in 1927 of methods for artificially inducing mutations by means of ionizing radiations and other mutagens opened the way for much new genetics research.
Modification of Mendel's principles developed as knowledge of the chromosomes increased; many discoveries have helped to account for apparent deviations from Mendelian ratios. For example, Mendel's studies emphasized genes that behave independently from one another during transmission to offspring. But we now know that genes are transmitted as constituents of chromosomes, each of which carries many different genes, which sheds light on the tendency of certain characteristics to appear in combination with one another (linkage). It also has been found that some characteristics are sex-linked, i.e., are transmitted by genes carried by the sex chromosomes (see sexsex, term used to refer both to the two groups distinguished as males and females, and to the anatomical and physiological characteristics associated with maleness and femaleness. ..... Click the link for more information. ); and that a non-sex-linked gene inherited from the father may differ in its expression from the same gene inherited from the mother, a phenomenon called "imprinting." Other research has shown that there may be multiple alleles (more than two alternative genes) for a given characteristic: the human blood groupsblood groups, differentiation of blood by type, classified according to immunological (antigenic) properties, which are determined by specific substances on the surface of red blood cells. ..... Click the link for more information. are determined by a combination of several possible alleles. It is apparent that homologous portions of paired chromosomes may be interchanged during meiosismeiosis , process of nuclear division in a living cell by which the number of chromosomes is reduced to half the original number. Meiosis occurs only in the process of gametogenesis, i.e., when the gametes, or sex cells (ovum and sperm), are being formed. ..... Click the link for more information. (crossing over) and that the interaction of many genes is responsible for determining many of the traits of individuals. Since the discovery (1953) of the structure of DNA, work on nucleic acidsnucleic acid, any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis. ..... Click the link for more information. has begun to explain how genes determine life processes by directing the synthesis of proteins. It has also explained mutations as alterations in gene or chromosome structure. It has been found, for example, that mutations in the form of repeated sequences of otherwise normal chemical bases, can grow in length with succeeding generations, in some cases causing diseases (e.g., myotonic muscular dystrophymuscular dystrophy , any of several inherited diseases characterized by progressive wasting of the skeletal muscles. There are five main forms of the disease. They are classified according to the age at onset of symptoms, the pattern of inheritance, and the part of the body ..... Click the link for more information. ) that increase in severity each time they are inherited.
Most of the knowledge of chromosome structure and the behavior of genes has come from studies of the vinegar, or fruit, fly (Drosophila melanogaster), which reproduces so rapidly that many generations can be studied over a short time. The work of T. H. Morgan and his associates on Drosophila was the basis of much of the early progress of genetics in the United States. Certain other small laboratory animals, plants, and microorganisms such as the E. coli bacteria are now used, also largely because of their ability to reproduce rapidly. For obvious reasons human beings are poor subjects for experimental genetic studies; however, much that aids understanding heredity in humans has been learned from the "lower" forms of life. Also, by tracing the appearance of certain abnormal characteristics (e.g., hemophilia, color blindness, and certain mental disorders and anatomical defects) and blood groups through a number of generations the hereditary pattern of these conditions has been established. The increasing ability of scientists to decode genetic information (see Human Genome ProjectHuman Genome Project, international scientific effort to map all of the genes on the 23 pairs of human chromosomes and, to sequence the 3.1 billion DNA base pairs that make up the chromosomes (see nucleic acid). ..... Click the link for more information. ) has led to a considerable expansion of knowledge about the nature and role of genes in humans and other organisms. Application of this knowledge has played an important role in the fields of gene therapygene therapy, the use of genes and the techniques of genetic engineering in the treatment of a genetic disorder or chronic disease. There are many techniques of gene therapy, all of them still in experimental stages. ..... Click the link for more information. , genetic engineeringgenetic engineering, the use of various methods to manipulate the DNA (genetic material) of cells to change hereditary traits or produce biological products. The techniques include the use of hybridomas (hybrids of rapidly multiplying cancer cells and of cells that make a ..... Click the link for more information. , and evolutionary studies, and has resulted in a better understanding of the genetic components of disease, physical characteristics, mental illness, and even personality.
The study of mutations, together with the analyses of population genetics, has been used to explain the mechanism of evolutionevolution, concept that embodies the belief that existing animals and plants developed by a process of gradual, continuous change from previously existing forms. This theory, also known as descent with modification, constitutes organic evolution. ..... Click the link for more information. . The elementary process of evolution is considered to be the changes in the frequency of occurrence of alleles in a population. Mutation, which causes the appearance of new alleles or changes the relative frequency of already existing alleles, is one important mechanism by which evolution occurs. Natural selection (see selectionselection. In Darwinism, the mechanism of natural selection is considered of major importance in the process of evolution. Popular formulations sometimes envisage a struggle for existence in which direct competition for mates or for various factors in the environment (e.g. ..... Click the link for more information. ), by affecting reproductive success, influences the frequencies of alleles and other genetic variants in successive generations. For example, if the presence of a particular allele makes a homozygous individual unable to mate, the allele may be eliminated from the population.
Genetic drift the random fluctuation in the frequency of an allele, resulting mainly from the vagaries of chance matingis also an evolutionary mechanism. Although in large populations drift varies only a little above and below a statistical mean, in small breeding populations an entire generation might, by chance alone, be born with the same genotype with respect to a particular allelic pair of genes, thus leading to either the elimination or dominance of a particular gene. Because fluctuations in the proportions of alleles are more significant in the gene pools of small, isolated breeding populations, genetic drift is a mechanism of species diversity and evolution in such groups.
See T. Beebe and J. Burke, Gene Structure and Transcription (1988); R. McKie, The Genetic Jigsaw (1988); G. L. Stine, The New Human Genetics (1988); G. W. Burns and P. J. Bottino, The Science of Genetics (1989); C. Wills, The Wisdom of the Genes (1989); G. Edlin, Human Genetics (1990); B. Lewin, Genes IV (1990).
1.the branch of biology concerned with the study of heredity and variation in organisms
2.the genetic features and constitution of a single organism, species, or group http://www.geneticalliance.org http://www.ornl.gov/TechResources/Human_Genome/ http://www.genetics.org http://ghr.nlm.nih.gov http://www.hgc.gov.uk
The science of biological inheritance, that is, the causes of the resemblances and differences among related individuals.
Genetics occupies a central position in biology, for essentially the same principles apply to all animals and plants, and understanding of inheritance is basic for the study of evolution and for the improvement of cultivated plants and domestic animals. It has also been found that genetics has much to contribute to the study of embryology, biochemistry, pathology, anthropology, and other subjects. See Biochemistry, Embryology
Genetics may also be defined as the science that deals with the nature and behavior of the genes, the fundamental hereditary units. From this point of view, evolution is seen as the study of changes in the gene composition of populations, whereas embryology is the study of the effects of the genes on the development of the organism. See Gene action, Population genetics
The field of molecular genetics describes the basis of inheritance at the molecular level. It focuses on two general questions: how do genes specify the structure and function of organisms, and how are genes replicated and transmitted to successive generations? Both questions have been answered. Genes specify organismal structure and function according to a process described by the central dogma of molecular biology: DNA is made into messenger ribonucleic acid (mRNA), which specifies the structure of a protein; the mRNA molecule then serves as a template for protein synthesis, which is carried out by complex machinery that comprises a particle called a ribosome and special adapter RNA molecules called transfer RNA. See Deoxyribonucleic acid (DNA), Ribonucleic acid (RNA), Ribosomes
The structure of DNA provides a simple mechanism for genes to be faithfully reproduced: the specific interaction between the nucleotides means that each strand of the double helix carries the information for producing the other strand. See Genetic code, Genetic engineering, Molecular biology, Mutation
a science that deals with the laws of heredity and variation of organisms. The most important aim of genetics is to work out methods of controlling heredity and hereditary variation in order to obtain the forms of organisms man needs or to control their individual development.
Main stages and trends. The fundamental laws of genetics were discovered by the Czech naturalist G. Mendel when he was crossing different species of peas (1865). However, the principal results of his experiments were not understood and appreciated by science until 1900, when the Dutch scientist H. de Vries, the German scientist C. Correns, and the Australian scientist E. Tschermak rediscovered the laws of inheritance of characters established by Mendel. The rapid development of genetics began at this time, confirming the principle of discreteness in hereditary phenomena and organization of genetic material and concentrating on the laws of inheritance of the characters and properties of parents by their offspring. The method of hybridological analysis played a large role in genetics; it consisted essentially of precise statistical description of the distribution of individual characters in a population of offspring obtained by crossing individuals especially selected for their hereditary qualities. Within a decade the results of hybridological analysis and cytologya science that studies the behavior of chromosomes during cell division (mitosis) and maturation of gametes (meiosis) and during fertilizationled to the creation of cytogenetics, which related the laws of character inheritance to the behavior of chromosomes during meiosis and substantiated the chromosomal theory of heredity and the theory of the gene as the material unit of heredity. The chromosomal theory explained the principle of segregation and the independent inheritance of characters in offspring and served as the basis for understanding many fundamental biological phenomena. The term gene, introduced by the Danish scientist W. Johannsen in 1909, was interpreted as the hereditary basis of a character. The work of the American geneticist T. H. Morgan (1911) and his many coworkers and students, chiefly C. Bridges, H. Muller, and A. Sturtevant, contributed greatly to the validation of the chromosomal theory of heredity. A major landmark in the development of heredity was the discovery of the mutagenic (heredity-altering) effect of X rays (the Soviet scientists G. A. Nadson and G. S. Filippov, 1925, and the American scientist H. Muller, 1927). After demonstrating the sharp increase in gene variation under the influence of external factors, the discovery led to the creation of radiation genetics. The research carried out on radiation and chemical mutagenesis (by the Soviet geneticists M. N. Meisel, 1928; V. V. Sakharov, 1933; M. E. Lobashev, 1934; S. M. Gershenzon, 1939; and I. A. Rapoport, 1943; and the English geneticist C. Auerbach, 1944) facilitated the study of the ultrastructure of the gene. Research on producing new forms of plants and microorganisms that had been altered hereditarily was of great practical value. The work of Soviet geneticists was important in the development of gene theory. A. S. Serebrovskii first posed the problem of the complex structure of the gene. Subsequently (1929 to 1931) he and his coworkers, especially N. P. Dubinin, experimentally demonstrated the divisibility of the gene and advanced a theory to explain its construction from subunits.
Genetics had a major role in the confirmation and elaboration of Darwins theory of evolution. Evolutionary genetics (including population genetics) investigates the genetic mechanisms of selection and the role of individual genes, genetic systems, and the mutation process in evolution. The Soviet geneticist S. S. Chetverikov made a fundamental contribution to population genetics in 1926 by combining in a single concept the ideas of Mendelism and Darwins theory of evolution. The development of evolutionary and population genetics was considerably advanced by the American scientist S. Wright and the English scientists J. Haldane and R. Fisher, who in the 1920s and 1930s laid the foundation for mathematical-genetic methods and the genetic theory of selection. Soviet scientists, especially N. P. Dubinin and D. D. Romashov, but also N. V. Timofeev-Resovskii, as well as the school of T. G. Dobzhansky in the USA, did much to advance experimental population genetics.
In the early stages of its development, genetics made a very important contribution to the theoretical substantiation of the methods of selection (the work of the Danish geneticist W. Johannsen, 1903, and the Swedish scientist H. Nielsson-Ehle, 1908). The unity of genetics and selection was most fully reflected in the work of the Soviet scientist N. I. Vavilov, who discovered the law of homologous series in hereditary variation and substantiated the theory that there are world centers of origin of all cultivated plants. Vavilov directed extensive research on all the worlds cultivated plants and their wild parents and on methods of breeding them. The names of G. D. Karpechenko and I. V. Michurin are associated with the theory of distant hybridization of plants. The Soviet geneticists M. F. Ivanov, P. N. Kuleshov, A. S. Serebrovskii, and B. N. Vasin made a major contribution to the study of the genetic basis of breeding animals. The Soviet scientist N. K. Koltsov was the first to clearly formulate (in 1927 and 1935) the matrical principle of reproduction of the molecular structure of hereditary material (chromosomes as hereditary molecules).
The use of microorganisms and viruses as objects of genetic research and the incorporation of the ideas and methods of chemistry, physics, and mathematics into genetics led to the appearance and rapid development of molecular genetics in the 1940s.
In the 1920s and 1930s, Soviet geneticists were leading the world in the study of heredity and variation. In 1939 and especially after the August meeting of the V. I. Lenin All-Union Academy of Agricultural Sciences (1948) the development of Soviet genetics decelerated, but after October 1964 it began to experience a period of multifaceted growth, which it is still undergoing.
There are many new trends in modern genetics that are of both theoretical and practical interest. The role of the genetic apparatus in ontogeny is a subject now being pursued with special vigor. This fact has helped to broaden the connections genetics has with embryology, physiology, immunology, and medicine. Human genetics, especially medical genetics, has become the most important branch. The genetic aspects of cancer control and premature aging are of major concern. Research is actively conducted on the genetics of animal and human behavior and on many other closely linked and interacting branches of genetics.
Extensive use is made of specially bred animals and plants, such as fruit flies, mice, rats, corn, or Arabidopsis, and strains of microorganisms, viruses, and cultures of various somatic cells in model genetic experiments. Biochemical and cytochemical methods, optical and electron microscopy, spectroscopy, cytophotometry, autoradiography, methods of local injury to cell organelles, and X-ray diffraction analysis are being increasingly employed. Mathematical-genetic methods are widely used to analyze the results of genetic experiments as well as to plan them.
Principal concepts and laws. Modern genetics regards heredity as the fundamental capacity of all organisms, inseparable from the concept of life, to duplicate in a series of successive generations similar types of biosynthesis and metabolism as a whole. This duplication guarantees the structural and functional continuity of living thingsfrom their intracellular apparatus to the morphological and physiological organization at all stages of individual development. Heredity and hereditary variationthat is, changes constantly arising in the genotypic bases of organismssupply material from which natural selection creates the different forms of life and ensures the progressive course of evolution. One of the fundamental tenets of modern genetics is that hereditary information about the development and properties of organisms is present chiefly in the molecular structures of the chromosomes contained in the nuclei of all the organisms cells and is transmitted from parents to progeny. The biochemical processes underlying the individual development of the organism are stimulated by information from the nuclei reaching the cytoplasmatic structures of the cell. Certain cell organelles, specifically chloroplasts and mitochondria, possess genetic autonomy, that is, they contain hereditary material. However, the nucleus is the dominant factor in heredity, as was shown, for example, in the experiments of the Soviet scientist B. L. Astaurov.
PATTERNS OF DISCRETE HEREDITY. One of the fundamental principles of genetics is the discreteness of the hereditary factors responsible for the development of characters and properties. The characters of the parents are not destroyed by crossing, and they are not mixed. These characters develop in hybrids of the first generation, either in a form typical of one of the parents or in an intermediate form, and they are again manifested in certain proportions in the following generations, a phenomenon demonstrated for the first time by G. Mendel. In crossing races of the garden pea that differ in the color of their cotyledons (yellow and green), Mendel observed that first-generation seeds had yellow cotyledons, and second-generation seeds obtained by self-pollination of first-generation plants had both yellow and green cotyledons. The ratio of the quantities of such seeds was 3:1. This phenomenon is called segregation. The character (yellow color of the cotyledon) that suppresses the development of the contrasting character in first-generation hybrids is called dominant, and the character suppressed (green color) recessive. Second-generation seeds with yellow cotyledons are genetically heterogeneous. A third of these seeds is constant with respect to the yellow color of the cotyledons, whereas the plants developing from the other two-thirds of the yellow seeds after self-pollination are again segregated by seed color in a 3:1 ratio. The green seeds are genetically homogeneous; after self-pollination the plants developing from them do not exhibit segregation and all produce only green seeds.
Mendel introduced letter symbols for convenience in analyzing the phenomena of inheritance of characters. Genes of the dominant characters are designated by capital letters, and those of the recessive characters by small letters. The hereditary basis of an organism constant with respect to some dominant character can be designated by the formula AA; the genetic formula of an organism with a recessive character is aa. The crossing of AA with aa organisms results in a hybrid form whose hereditary basis can be expressed by the formula Aa. The letters A and a designate, accordingly, the genes that influence the development of the same characterin the example given, the color of the cotyledons. Organisms possessing only genes determining the development of a dominant (AA) or recessive (aa) character are called homozygotes, and organisms with both kinds of genes (Aa) are called heterozygotes. Genes occupying the same position in homologous chromosomes and influencing the development of the same characters are called allelic genes. The phenomenon of segregation of characters of hybrid (heterozygous) organisms is based on the fact that the sex cells (gametes) of hybrids possess only one of the two allelic genes (A or a) obtained from their parents. This is known as the principle of purity of gametes, which reflects the discreteness of the structure of hereditary material. The purity of gametes is due to the divergence in meiosis of homologous chromosomes and of the allelic genes contained within them in the offspring cells, whereas the numerical correlations of types in the offspring produced by the crossing of heterozygous individuals are due to the equal probability of encountering gametes and the genes contained within them.
If the analysis is made solely from a single character, two types of offspring are found, one with a dominant character, the other with a recessive (in the ratio of 3:1). However, if the genetic structure of the organisms is taken into account, three types of offspring can be distinguished: 1AA (homozygous for the dominant character), 2Aa (heterozygous), and laa (homozygous for the recessive character). Mendels analysis of the inheritance of two different characters, such as the color of the cotyledons and the shape of the pea seeds, showed that segregation takes place in the offspring of hybrid (heterozygous) individuals according to both characters, and both are combined in the second generation offspring independently of each other. Since two types of offspring arise in the ratio of 3:1 after segregation by each character, then in the case of two independently inherited characters there will be four types of offspring in the second generation in the ratio of (3 + 1) (3 + 1) = 9 + 3 + 3 + 1; that is, nine-sixteenths of the offspring with both dominant characters, three-sixteenths with the first dominant and the second recessive characters, three-sixteenths with the first recessive and the second dominant characters, and one-sixteenth with both recessive characters. In case of complete dominance, the proportion of types of offspring produced by crossing individuals differing in any number of characters can be calculated from the formula for expansion of the binomial (3 + l)n, where n is the number of pairs of genes by which the crossed parental forms are distinguished. The independence of inheritance, or free combination, is peculiar to those characters for which the genes lying in different (non-homologous) chromosomes are responsible. Thus, the cause of independent inheritance is the independent divergence of nonhomologous chromosomes in meiosis.
The ensuing detailed analysis of the patterns of heredity showed that the set of characters of an organism (phenotype) does not always correspond to its hereditary constitution (genotype) because even when the hereditary basis is identical, characters can develop in different ways under the influence of a variety of external conditions. Hereditarily determined characters may not be manifested in the phenotype, either because they are recessive or because external factors have been at work. If the phenotype of an individual is accessible to direct observation, its genotype can be judged from a study of the offspring obtained in certain crossings. The individual development of organisms and formation of their characters are realized on the basis of the genotype, depending on the prevailing environmental conditions.
One of the basic theories of genetics is the chromosomal theory of heredity. The cornerstone of this theory is the fact that the development of the properties and characters of an organism is determined by strictly localized segments of chromosomes, genes arranged in a linear order. The doubling of chromosomes also guarantees the doubling of genes and their transmission to each newly created cell. The genes on a single chromosome constitute a single linkage group, and they are transmitted together; the number of linkage groups is equal to the number of chromosome pairs, which is constant for each species of organism. The characters depending on linked (that is, arranged in one chromosome) genes are also inherited together. Linked inheritance of characters can be disrupted by crossing-over, which results in the redistribution of genetic material during meiosis between homologous chromosomes. The closer the genes are to one another, the smaller the probability of this recombination. The frequency of recombination is also influenced by the sex and physiological state of the individuals and by external conditions such as temperature. The frequency of recombination is a measure of the distance between the genes. Methods for determining the position of genes in a chromosome are based on this fact, and so-called genetic maps of chromosomes have been constructed for several plants and animals. Cytological maps of chromosomes have also been constructed for Drosophila and corn; in these maps the genes are localized in definite parts of chromosomes visible under a microscope. Genetic and cytological maps complement and confirm each other.
It has been proved that a single gene can influence not one but many characters (pleiotropy) and, moreover, the development of each character depends on many genes (polymeria) rather than on one. It has also been proved that the function of a gene and its influence on the phenotype varies with the physical position of the gene in the genetic system (position effect), set of other genes (genotypic environment), and external conditions. The phenotypic expression of a gene (expressivity), like its penetrancethe presence or absence of a character controlled by the particular genemay vary both with external conditions and with the genotype. Genes may change or mutate under the influence of various external factors. The elementary units of a gene are also capable of independent mutation. All these facts are indicative of the complexity of the physical structure of the gene as it has evolved during the development of life on earth.
After the discovery of the molecular bases of the organization of the hereditary structures and processes underlying the transmission of hereditary information in the cell and the organism and in generations of cells and organisms, it was found that genes control the process of protein synthesis in the cells and that gene mutations, or changes in the chemical structure of genes, alter the chemical structure of proteins (in some cases causing the substitution of one amino acid for another). The giant polymer deoxyribonucleic acid (DNA), the most important structural constituent of the chromosomes in all organisms except for certain viruses that contain ribonucleic acid (RNA), is the physical carrier of genetic information.
When the DNA molecules are duplicated during cell division, the offspring molecules, with the help of specific enzymes, are constructed on the parent molecules as on a template, and they exactly reproduce the parent molecules. The genetic code recorded in the molecular structures (sequence of nucleotides) of DNA determines the arrangement of the amino acids in the protein molecule. Information is transmitted from DNA to the proteins being synthesized by RNA. The RNA molecules are constructed on DNA and are complementary to it; as a result, the coding structure of DNA is reproduced in the RNA molecules. The cell has several types of RNA: information (iRNA), transport (tRNA), and ribosomal (rRNA), which differ from one another in molecule size, in structure, and in function. The arrangement of the amino acids in the protein molecules is controlled by the high-polymeric iRNA; biosynthesis of protein takes place in cytoplasmatic ribonucleoprotein (protein + rRNA) structures, or ribosomes, by means of the enzymes (aminoacyl-rRNA-synthetases) and the energy of adenosine triphosphate (ATP) stored in the mitochondria. The amino acids are transported to the ribosomes by the comparatively low-polymeric tRNA. The structure of iRNA determines the location and arrangement of the amino acids in the protein moleculesthe primary structure of the protein molecules and their main properties. The gene, or portion of a DNA molecule, that controls the synthesis of the polypeptide chains of some proteins is called a structural gene. The structure and functions of many structural genes (cistrons) have been thoroughly studied in several microorganisms (for example, in Bacillus coli and Salmonella) and bacteriophages. The structural genes controlling the synthesis of enzymes in a definite sequence of reactions were found to be linked together in blocks, or operons. There are structures (so-called operators) that switch on the synthesis of iRNA by structural genes. The operators are controlled, in turn, by regulator genes. Thus, the genes constitute a complex system that closely coordinates the process of biosynthesis in the cell and in the organism as a whole. Only some of the genes in the cells are functionally active, and the others are repressed. The spectrum of proteins synthesized in the cell changes because of the regular succession of states of gene activity and repression. For example, the embryonal type of hemoglobin is synthesized in the human fetus, but by the end of the first year of life it begins to gradually be replaced by the normal adult type of hemoglobin. The dynamics of the active and repressed states of the genetic apparatus have also been observed directly by microscopic and cytochemical methods in giant chromosomes in cells of the salivary glands of certain two-winged fly larvae (Drosophila and Chironomus). Every stage in the development of an organism exhibits a characteristic pattern of synthetic activity of the chromosomes; certain segments are highly active and synthesize RNA, and others are functionally inactive at these stages but become active at others. It was found that in some cases hormones regulate the functional activity of the genetic apparatus. The study of the genetic aspects of ontogeny is one of the most urgent tasks in modern biology.
The genetic apparatus functions in close coordination with the extrachromosomal or extranuclear constituents of the cell. Many facts testify to the importance of cytoplasm in the development of the organism and, in some cases, in inheritance. For example, male sterility in corn and other plants caused by the death of pollen is the result of the interaction of certain cytoplasmic and nuclear factors. The phenomenon of plastic heredity has long been known. The properties of cytoplasm play a major role in interspecific crossings, being largely responsible for the viability and fertility of the hybrids. The properties of cytoplasm are controlled, in turn, by the nuclear apparatus in which changes during crossings lead to changes in the properties of cytoplasm.
Patterns of mutation. The hereditary variety of individuals is created both by the recombination of genes during crossings and as a result of changes in the genes themselves, that is, by mutations. The following main types of mutations are distinguished: gene, chromosomal, and point. Gene mutations include polyploidy; this is an increase in the number of chromosomes to a multiple of the basic or haploid (n) set, resulting in triploids, tetraploids, and so forth, that is, organisms with triple (3n), quadruple (4n), etc. the number of chromosomes in the somatic cells. Amphidiploidythat is, double the number of chromosomes of each parent in distant (interspecific and intergeneric) hybridshas great evolutionary significance in that it ensures the normal course of meiosis and restoration of fertility in usually sterile hybrids. This was demonstrated for the first time by G. D. Kar-pechenko in 1927, when he produced cabbage-radish hybrids. Many species of cultivated plants are natural amphidiploids. For example, the 42-chromosome wheats are complex amphidiploids (hexaploids) bearing the genomes of the wild einkorn wheat and two species of goat grass related to the wheat of wild grasses; each of these species has a diploid set of chromosomes (2n) equal to 14. A hybrid (amphidiploid) origin has also been proved for oats, cotton, tobacco, sugar beets, plums, and other cultivated and wild plants. Some of these species were artificially resynthesized (for example, the plum by the Soviet geneticist V. A. Rybin) by crossing the parental forms and then using experimental polyploidy. Gene mutations also include aneuploidy, or heteroploidy, that is, an increase or decrease in the number of chromosomes of one or several homologous pairs. This changes some of the characters of the organism, and in man it may cause serious diseases.
Mutations in the form of chromosomal aberrations include various types of reorganization of chromosomes and redistribution of their genetic material within the genome. One type is translocation, that is, the reciprocal exchange of nonhomologous segments between chromosomes; another is inversion, that is, the turning of a segment of a chromosome 180, thereby altering the arrangement of the genes in the chromosome. In deletion, part of a chromosome is lost; and in duplication particular segments of chromosomes are doubled. Many of these changes have a more or less significant effect on the phenotype, indicating that the action of genes is dependent on their position in the genome.
Point mutations are particularly significant in evolution and in breeding. Point mutations include all the changes that do not result in structural disruptions of individual chromosomes that can be detected by cytological methods. This group embraces tiny deletions, duplications, and inversions as well as changes in the hereditary code at the molecular level (true gene mutations). It is frequently impossible to draw a line between point and gene mutations. Analysis at the molecular level of gene mutations in viruses shows that they are caused by the loss or insertion of individual nucleotides in the DNA molecule or by the replacement of some nitrogenous bases with others (transitions and transversions) during the replication of DNA.
Mutability is inherent in all genes in both the sex and somatic cells of organisms. Spontaneous mutations of individual genes are rare. The average rate of occurrence is one mutation per 100,000 to 200,000 or even 1 million genes, and sometimes the ratio is even lower. This is of definite evolutionary significance because it makes for the stability of the hereditary system, without which life could not exist. Stability is ensured particularly by the presence of enzymes that help to repair the breaks that arise in the hereditary structures. Genes do not all mutate with the same frequency this indicates that mutability depends on both the structure of the gene and the remaining genotype. The physiological state of the cell and of the integral organismspecifically its ageand many external conditions strongly influence the rate of mutagenesis. Most mutations are recessive, and they generally have an adverse effect, rendering the organism completely or partly nonviable.
All kinds of ionizing radiation, ultraviolet rays, and several chemical substances possess powerful mutagenic action, that is, the capacity to increase the frequency of mutations many times. All these agents are widely used in genetic and selective practice to obtain mutant forms of microorganisms and plants. Mutations are not forms of adaptation, and they are inadequate to the factors operating on the organism. The same factors may result in mutation of different genes; on the other hand, the same genes may mutate when exposed to different factors. The principle of nondirectivity of the mutation process is based on this phenomenon.
However, in both natural and artificially induced mutagenesis, especially that caused by chemical mutagens, there is a known specificity of the spectrum of resulting mutations due to both the peculiar mechanism of action of the mutagen and the characteristics of the genotype of organisms. For example, the treatment of dividing cells with the alkaloid colchicine leads to polyploidization of the cells, and it is widely used to obtain new forms of plants by the methods of experimental polyploidy. Ultraviolet rays and chemical mutagens most often induce gene mutations, whereas neutrons cause many chromosomal aberrations. The specificity of mutation of certain genes has been found during different mutagenic actions. Experiments on viruses and bacteria have revealed that certain chemical mutagens are selectively active against some nitrogen bases in the DNA molecule. Thus, genetics comes close to solving the problem of control of mutation at the molecular level. However, the fundamental problem of modern sciencedirected induction of mutations in complex multicellular organismsstill awaits a solution.
Genetics and evolution. Mendels discovery of the laws of segregation showed that recessive mutations do not disappear, but remain in populations in the heterozygous state. This overcame one of the most serious objections to Darwins theory of evolution, which was voiced by the English engineer F. Jenkin. The latter maintained that the magnitude of useful hereditary change that takes place in an individual will decrease in the following generations and gradually approach zero.
Genetics validated the view that the genotype determines the standard for an organisms reaction to the environment. Within this standard, environmental conditions may influence the individual development of organisms by altering their morphological and physiological properties, that is, by causing modifications. However, these conditions do not cause adequate (that is, corresponding to the environment) changes in the genotype. Hence the modifications are not inherited, although the actual possibility of their arising under the influence of environmental conditions is determined by the genotype. It is in this sense that genetics answered negatively the question of whether the characters acquired in the course of individual development can be inheritedan answer of enormous significance both in confirming Darwins theory of evolution and in breeding.
Research has showed that natural populations are saturated with mutations, mainly recessive, which persist in the heterozygous state concealed in the normal phenotype. In unlimitedly large populations with free crossing and no pressure of selection, the concentration of allelic genes and corresponding genotypes (AA, Aa, aa) is in a definite balance described by the formula of the English mathematician H. Hardy and the German physician W. Weinberg:
p2AA + 2 pqAa + q2aa,
where the coefficients p and q are the concentrations of the dominant and recessive genes expressed in fractions (p + q = 1). In real natural populations, the concentration of mutant genes depends mainly on the pressure of selection that determines the fate of the carriers of mutations according to their influence on the viability and fertility of individuals under concrete environmental conditions. The carriers of unfavorable mutations are eliminated by selection. However, many mutations that are unfavorable or even lethal in a homozygous state may in a heterozygous state increase the viability of the carriers, and as a result they persist in the populations at a certain level. Since the same mutations influence the fitness of organisms differently under different environmental conditions and directions of selection, they serve as the means used by selection to create intraspecific polymorphism, which ensures the fitness of a species and its evolutionary plasticity under widely varying habitat conditions. The mutations within the normal phenotype create a mobilization reserve of hereditary variation (I. I. Shmal-gauzen) that selection can act upon following a change in the conditions of a species existence. Because mutations may affect the development of characters differently according to the genotypic peculiarities of the organismthat is, the genotypic environment affected by the mutated gene selection evaluates the phenotypes of the individuals and includes in its sphere of activity not the individual mutations as such but the integral genotypes, picking up those that ensure the most delicate adaptation of organisms to the environment.
Genetic research has also explained the role of the mutation process, isolation, migration, hybridization, and the so-called genetic-automatic processes in the evolutionary divergence of populations and mechanisms of speciation. Thus, the findings of genetics have confirmed the fundamental ideas of Darwins theory of evolution and at the same time uncovered new laws of heredity and variation, on the basis of which selection creates an infinite number of forms of living organisms with a remarkable ability to adapt to the environment.
Practical applications. The laws discovered by general and population genetics and the methods of evaluating the genetic parameters of populations are the foundation of the modern theory of selection and breeding. Having found that breeding is effective only when it relies on the hereditary variety of individuals in a population and that the phenotype does not always match the genotype, genetics substantiated the need to evaluate with appropriate methods and practical techniques the hereditary qualities and variety of selected organisms and equipped breeders. For example, the evaluation of the hereditary qualities of parents from the economically important characters of their offspring, long practiced by the best animal breeders, was scientifically justified by genetics as an essential technique in the breeding of pedigreed stock and particularly useful in connection with the widespread method of artificial insemination. The methods of individual plant breeding are also based on the concepts of genetics regarding pure strains, homo- and heterozygosity, and nonidentity of the phenotype and genotype. The genetic laws of independent inheritance and free combination of characters in the progeny were the theoretical bases for hybridization and crossing, which along with selection constitute the principal methods of breeding. The Soviet breeders P. P. Lukianenko, V. S. Pustovoit, V. N. Mamontova, V. Ia. Iurev, V. P. Kuzmin, A. L. Mazlumov, M. I. Khadzhinov, and P. I. Lisitsyn created a remarkable variety of grains and industrial and other crops based on hybridization and selection. Of great importance in increasing the effectiveness of plant breeding are N. I. Vavilovs law of homologous series, his teaching on the genetic centers of origin of cultivated plants, and his theory of distant ecological and geographic crossings and immunity.
The methods of breeding individual animal and plant species are improved by the work done on the individual genetics of these species. For example, minks or Karakul sheep of different colors could not be bred without a knowledge of the laws of inheritance of colors in these animals. In minks there is a genetic synthesis of the natural color of the fur with sapphire, platinum, and other nonnatural colors that is based on the genetic laws of independent inheritance and interaction of genes. Distant hybridization is widely used to create new plant varieties. It has produced many valuable varieties of fruit-plant (I. V. Michurin) and wheat-couch grass hybrids (N. V. Tsitsin and G. D. Lapchenko) and some kinds of hybrid winter wheat. Distant hybridization is also successfully used in the breeding of such plants as potatoes, beets, some trees, and tobacco. The phenomenon of cytoplasmic male sterility is used in the breeding of corn, wheat, sorghum, and other crops. The methods of experimental polyploidy are being increasingly applied in the creation of economically valuable kinds of crops. Highly valuable triploid sugar-beet and buckwheat hybrids, triploid seedless watermelon, and polyploid rye, clover, and mint were created by these methods.
Ionizing radiation and chemical agents are being used more and more to induce mutations, especially in microorganisms. Mutant strains of the producers of several antibiotics, amino acids, enzymes, and other biologically active substances that are far more productive than the original strains are already in existence. Artificial mutagenesis used in plant selection in the USSR as early as the end of the 1920s (L. N. Delone, A. A. Sapegin, and others) is now widely employed in selection work in several other countries. Artificially obtained mutant forms were used to create high-yielding strains of barley, wheat, rice, oats, peas, soybeans, beans, lupines and other species now being produced. By greatly increasing the hereditary variability of plants, the methods of experimental polyploidy and artificial mutagenesis accelerate the process of breeding and make it more efficient. However, this does not minimize the role of selection and hybridization. The value of the old methods of breeding varieties and strains, combined with the new techniques, based on advances in genetics, is steadily increasing, especially in the selection of animals, where experimental polyploidy and mutagenesis are still not used. Elaboration of the theory and methods of evaluation, selection, and breeding of animals and plants, as well as of the systems for raising them in the best way possible, is still an important task.
Advances in genetics have been the basis for the use of methods of genetically regulated heterosis, which have produced yields of hybrid corn 30-40 percent higher than those of the original varieties, as well as sorghum and other crops. These methods have also been used with farm animals such as swine and especially chickens. (The best hybrid hens excel purebred hens or crossbred hybrids in egg production, size of eggs, and return from feed costs.)
Genetics is playing an increasingly important role in the study of human heredity and in the prevention and treatment of hereditary diseases. It has also made a major contribution to the study of the dialectical-materialist concept of the world by showing that heredity, the fundamental property of life, is based on the complex physicochemical structure of the chromosomal apparatus, which was created in the course of evolution to store and transmit genetic information. In doing so, genetics provided one more proof of the interrelationship of the physicochemical and biological forms of the organization of matter with the unity of the material world. It showed that all genetic phenomena and processes, including hereditary variation, are determined. The dialectically contradictory unity of the phenomena of heredity and hereditary variation has been explained by the behavior and peculiarities of change in the structure of the chromosomes and the genes they contain during crossings and by the reaction of genetic material to external influences or to extracellular conditions. Genetics has also showed that the main internal contradiction between heredity and hereditary variation, resolved in the process of mutation, recombination during hybridization, and selection, is the motive force of evolution. Genetics has confirmed Darwins theory of evolution and helped to elaborate it. Having uncovered the material basis of hereditary phenomena, genetics by virtue of the very logic of development of natural science demonstrated that all genetic phenomena and processes obey the laws of dialectical motion. In developing the theory of heredity and variation, Soviet genetics rests firmly on the foundations of dialectical materialism and Marxist-Leninist philosophy.
Centers of research and publications. The main centers of genetic research in the USSR are, in Moscow, the Institute of General Genetics of the Academy of Sciences of the USSR (AN SSSR), the Institute of Biology of Development of the AN SSSR, the Institute of Molecular Biology of the AN SSSR, the Department of Chemical Genetics of the Institute of Chemical Physics of the AN SSSR, the Radiobiology Department of the Institute of Atomic Energy of the AN SSSR, and the Institute of Medical Genetics of the Academy of Medical Sciences of the USSR; the Institute of Cytology and Genetics of the Siberian Department of the AN SSSR in Novosibirsk; the Institute of Genetics and Cytology of the Academy of Sciences of the Byelorussian SSR in Minsk; the Institute of Cytology of the AN SSSR in Leningrad; the Institute of Genetics and Selection of Industrial Microorganisms of the Main Administration of the Microbiological Industry, Molecular Biology, and Genetics Sector of the Academy of Sciences of the Ukrainian SSR in Kiev; and the genetics departments of Moscow State, Leningrad State, and other universities. The N. I. Vavilov All-Union Society of Geneticists and Breeders was organized in 1965 with local branches. Genetics is taught in all the universities and medical and agricultural schools in the USSR. Research in genetics is also intensively conducted in the other socialist countries, as well as in Great Britain, India, Italy, the USA, France, the Federal Republic of Germany, Sweden, and Japan. International genetics conferences are held every five years.
The main source that regularly publishes articles on genetics is the journal Genetika of the AN SSSR (since 1965). The Academy of Sciences of the Ukrainian SSR issues the journal Tsitologiia i genetika (since 1967). Articles on genetics are also published in many biological journals, such as Tsitologiia (since 1959), Radiobiologiia (since 1961), and Molekuliarnaia biologiia (since 1967).
Outside the USSR articles on genetics are published in dozens of journals and annuals, such as Annual Review of Genetics (Palo Alto, since 1967), Theoretical and Applied Genetics (Berlin, since 1929), Biochemical Genetics (New York, since 1967), Molecular and General Genetics (Berlin, since 1908), Heredity (Edinburgh, since 1947), Genetical Research (Cambridge University Press, New York, since 1960), Hereditas (Lund, since 1920), Mutation Research (Amsterdam, since 1964), Genetics (Brooklyn, New York, from 1916; today, Austin, Texas); Journal of Heredity (Washington, D. C., since 1910), Canadian Journal of Genetics and Cytology (Ottawa, since 1959), Japanese Journal of Genetics (Tokyo, since 1921), Genetica Polonica (Pozna, since 1960), and Indian Journal of Genetics and Plant Breeding (New Delhi, since 1941).
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Genetics | Article about genetics by The Free Dictionary