the property characteristic of all living organisms, by which identical traits and developmental characteristics are repeated in a number of generations. Heredity depends on the transmission from one generation to another during reproduction of the material structures of the cell that contain programs for the development of new individuals.
The continuity of the morphological, physiological, and biochemical organization of living things and the character of their individual development, or ontogeny, are ensured by heredity. As a general biological phenomenon, heredity is the most important condition for the existence of differentiated forms of life. Different species would be impossible without a relative continuity in the traits of organisms, even though this continuity is disrupted by mutation, which gives rise to intraspecies differences between organisms. Embracing the most varied traits in all stages of ontogeny, heredity is manifested in the laws of inheritance of traits—that is, in the principles governing the transmission of characteristics from parents to offspring.
The term “heredity” is sometimes applied to the transmission from generation to generation of infectious principles (infective heredity) or of learning skills, education, and traditions (social or signal heredity). However, the broadening of the concept of heredity beyond its biological and evolutionary essence is debatable. Only in cases where infectious agents are capable of interacting with the host cells to the point where they are incorporated into their genetic apparatus is it difficult to separate infective from normal heredity. Conditioned reflexes are not inherited but are developed by each generation, although heredity undoubtedly plays a role in the speed with which they and various behavioral characteristics are reinforced. Thus, biological heredity is a component of social heredity.
Explanations of the phenomena of heredity that date from antiquity (Hippocrates, Aristotle) are only of historical interest. Until the discovery of sexual reproduction, the concept of heredity could not be accurately defined and linked with certain parts of the cell. Data on the laws of heredity were obtained by the mid-19th century from numerous experiments with plant hybridization (for example, J. G. Kölreuter’s work). In 1865, G. Mendel published a clear, mathematical generalization of the results of his experiments on the hybridization of peas. These generalizations were subsequently called the Mendelian laws, and they became the foundation of Mendelism, the teaching on heredity. At about the same time, attempts were made to gain a theoretical understanding of heredity. In the book The Variation of Animals and Plants Under Domestication (1868), C. Darwin advanced his “provisional hypothesis of pangenesis,” according to which the rudiments of all the cells of the organism (the gemmules) separate from the cells and, moving through the bloodstream, settle in the sex cells or in structures involved in asexual reproduction, such as buds. This implies that the sex cells and buds consist of a vast number of gemmules. When the organism develops, the gemmules are transformed into the same type of cells from which they were formed. The pangenesis hypothesis combined ideas of unequal value: the concept of the presence in the sex cells of special particles that determine the subsequent development of the individual, and the notion that these particles are transported from the somatic to the sex cells. The concept of special particles was correct and led to modern ideas regarding corpuscular heredity. But the second notion, which laid the foundation for the idea of the inheritance of acquired characteristics, was wrong. F. Galton, K. W. von Nageli, and H. de Vries also elaborated speculative theories of heredity.
The most detailed speculative theory of heredity was proposed in 1892 by A. Weismann. Drawing on the information then available on fertilization, he recognized the presence in the sex cells of a special substance, a carrier of heredity, or germ plasm. Weismann regarded the visible structures in the cell nucleus, the chromosomes (idants), as the highest units of germ plasm. According to his theory, idants consist of ids arranged in a line of granules. Ids consist of determinants, which determine the various kinds of cells that arise during the development of the individual, and biophores, which are responsible for the individual properties of the cells. An id contains all the determinants needed to construct the body of an individual of a given species. Germ plasm is found only in sex cells, not in somatic, or body, cells. To account for this radical difference between the two types of cell, Weismann assumed that during the cleavage of a fertilized egg, the main supply of germ plasm (that is, of determinants) enters one of the first cleavage cells, which becomes the parent cell for embryonic development. Only some of the determinants enter the remaining cells of the embryo during “unequal inheritance divisions.” Finally, the cells contain determinants of one kind, which determine their nature and properties. The essential property of germ plasm is its great continuity. Although Weismann’s theory was erroneous in many details, his idea concerning the role of chromosomes and the linear arrangement of the elementary units of heredity proved to be correct and anticipated the chromosomal theory. The logical conclusion to be drawn from Weismann’s theory is that acquired characteristics are not inherited.
All the speculative theories of heredity contained some elements that were subsequently confirmed and more fully developed in the science of genetics, which developed in the early 20th century. Among the most important contributions of the speculative theories is the concept of the segregation in the organism of individual traits or properties, the inheritance of which can be analyzed by appropriate methods, and the idea that these properties are determined by special, discrete units of heredity localized in a cell structure (the nucleus). Darwin called these units gemmules; de Vries, pangenes; and Weismann, determinants. The term “gene,” which was suggested by W. Johannsen in 1909, is widely accepted in contemporary genetics.
Attempts to discover the laws of heredity by using statistical methods fall into a special category. F. Galton, one of the founders of biometrics, used statistics to calculate correlations and regressions in order to establish the relationship between parents and offspring. He formulated a number of laws of heredity (1889), including atavism, or a throwback to the organism’s ancestors, and ancestral heredity, or the share of ancestral heredity in the heredity of the offspring. The laws are statistical, apply only to populations of organisms, and do not reveal the essence and causes of heredity, which could be disclosed only by the experimental study of heredity by different methods, especially hybrid analysis, the foundations of which were laid by Mendel.
The laws of inheritance of qualitative characteristics assert that in a monohybrid, the difference between crossed forms depends on only one pair of genes; in a dihybrid, on two; and in a polyhybrid, on many. Analysis of the inheritance of quantitative characteristics failed to provide a clear-cut picture of segregation. This led to the identification of a special, “blending inheritance,” which was explained by the mixing of the inherited plasms of the crossed forms. Subsequently, hybrid and biometric analysis of the inheritance of quantitative characteristics showed that even blending inheritance involves discrete traits, and inheritance of quantitative traits is polygenic. In polygenic inheritance it is difficult to detect segregation, because of the involvement of many genes whose effect on a trait is complicated by the strong influence of environmental conditions. Thus, although traits can be categorized as qualitative or quantitative, the terms “qualitative” and “quantitative” heredity are not justified, because both types of heredity are fundamentally the same.
The development of cytology led some scientists to raise the question of the material basis of heredity. Relying on their study of fertilization, O. Hertwig (1884) and E. Strasburger (1884) were the first to suggest that the nucleus functions as the carrier of heredity. T. Boveri (1887) discovered the individuality of chromosomes and hypothesized that they were qualitatively different. He and E. van Beneden (1883) observed that the number of chromosomes is halved during the formation of sex cells in meiosis. The American scientist W. Setton (1902) offered a cyto-logical explanation of Mendel’s law of the independent inheritance of traits. The chromosomal theory of heredity was substantiated beginning in 1911 by T. H. Morgan and his school, who demonstrated an exact correlation between the genetic and cytological data. In experiments on the fruit fly Drosophila they found an exception to the independent distribution of characters —linked inheritance, a phenomenon explained by the linkage of genes, or the location of the genes responsible for a particular trait on a single pair of chromosomes. Study of the frequency of recombinations between linked genes as a result of crossing-over made it possible to construct maps showing the location of genes on the chromosomes. The number of groups of linked genes equalled the number of pairs of chromosomes in a given species.
The most important evidence supporting the chromosomal theory of heredity came from a study of sex-linked inheritance. In the chromosome sets of several animal species cytologists discovered special sex chromosomes that distinguished females from males. In some cases the females have identical sex chromosomes (XX), and the males have different ones (XY). In other cases the males have two identical chromosomes (XX or ZZ), and the females have different ones (XY or ZW). The sex with identical sex chromosomes is called homogametic, and the sex with different ones, heterogametic. In some insects, including Drosophila, and in all mammals the females are homogametic, and the males, heterogametic. The opposite is true in birds and butterflies. In the fruit fly Drosophila the inheritance of several traits is associated strictly with the transmission of X chromosomes to the offspring. The female fly, for example, can carry a recessive gene for white eyes. Because the female fly is homozygous for this gene, which is located on the X chromosome, white eyes are transmitted to all the male offspring, who obtain their X chromosome from their female parent. If the female parent is heterozygous for a recessive sex-linked gene, the trait is transmitted to half the male progeny. In species where the males are homogametic (XX or ZZ) and the females heterogametic (XY or ZW), the males transmit sex-linked traits to their female offspring, who obtain their X (or Z) chromosome from the male parent.
Sometimes XXY females and XYY males appear as a result of the nondisjunction of sex chromosomes. It is also possible for X chromosomes to be joined at the ends. In this case, the females transmit linked X chromosomes to their female offspring, in whom sex-linked traits are expressed, but the male offspring are like the male parent (hologenic inheritance). If the inherited genes are located on the Y chromosome, the traits determined by them are transmitted only through the male line, from father to son (holandric inheritance). The chromosomal theory uncovered the intracellular mechanisms of heredity, provided an accurate and consistent explanation for all the phenomena of inheritance in sexual reproduction, and accounted for changes in heredity—that is, mutation.
The paramount role of the nucleus and chromosomes in heredity does not preclude the transmission of some traits through the cytoplasm, which contains structures capable of replication (cytoplasmic heredity). Units of cytoplasmic (non-chromosomal) heredity differ from those of chromosomal heredity in that they do not separate during meiosis. Therefore, in nonchromosomal heredity the offspring reproduces the traits of only one of the parents (generally the female). Thus, a distinction is made between nuclear heredity (sometimes called chromosomal heredity), which is associated with the transmission of hereditary traits located in the chromosomes of the nucleus, and extranuclear heredity, which depends on the replication of structures in the cytoplasm. Nuclear heredity is also associated with vegetative (asexual) reproduction, but it does not result in the redistribution of genes that occurs in sexual reproduction. It ensures a constant transmission of traits from generation to generation that is disrupted only by somatic mutations.
The availability of new physical and chemical methods, as well as the use of bacteria and viruses in research, sharply increased the capacity of genetic experiments to solve problems and led to the study of heredity at the molecular level and the rapid development of molecular genetics. N. K. Kol’tsov was the first to advance and substantiate the idea that heredity has a molecular basis. He proposed the matric method of reproducing “hereditary molecules” (reported in 1927 and published in 1928 and 1935). The genetic role of deoxyribonucleic acid (DNA) was experimentally demonstrated in the 1940’s, and its molecular structure and the principles of coding genetic information were established in the 1950’s and 1960’s.
The concept of the gene became more profound and more precise as the study of genetics at the subcellular and molecular level continued. In experiments on the inheritance of different traits the gene was postulated as the elementary, indivisible unit of heredity and was regarded in the light of cytological data as a separate part of the chromosome. At the molecular level, the gene was considered an integral part of the DNA molecule, capable of replication and possessing a specific structure in which the program for the development of one or more traits was coded. In the 1950’s, the American geneticist S. Benzer showed that in microorganisms every gene consists of several different parts capable of mutating and crossing over. This confirmed the idea developed in the 1930’s by A. S. Serebrovskii and N. P. Dubinin, who concluded from genetic analysis that the gene has a complex structure.
In 1967–69, viral DNA was synthesized in vitro and the gene of yeast alanine transport RNA was chemically synthesized. The heredity of somatic cells in vivo and in tissue culture became a new field of research. Scientists discovered that it is possible to hybridize experimentally the somatic cells of different species. The phenomena of heredity became key factors in many practical questions and in understanding certain biological processes.
The role of heredity in evolution was clear to Darwin. The establishment of the discrete character of heredity removed one of the important objections to Darwinism: the crossing of individuals in whom hereditary changes have appeared must “dilute” these changes or weaken their expression. According to Mendel’s laws, however, the changes are not destroyed or blended but reappear in the offspring under certain conditions. In populations the phenomena of heredity appeared to be complex processes based on crosses of individuals, as well as on selection, mutations, and genetic-automatic processes. The first to point this out was S. S. Chetverikov (1926), who demonstrated experimentally the accumulation of mutations within a population. I. I. Shmal’gauzen (1946) hypothesized that a “mobilization reserve of hereditary mutation” was the material for the creative activity of natural selection under changing environmental conditions. The significance of different types of mutations in evolution was demonstrated.
Evolution is defined as gradual and repeated change in the heredity of a species. At the same time, heredity, which ensures the continuity of species organization, is a fundamental property of life that has evolved over a long period of time and that is related to the physicochemical structure of the elementary units of the cell, particularly its chromosomal apparatus. The principles of the organization of this physicochemical structure (the genetic code) appear to be universal for all living things and are regarded as the most important attribute of life.
Ontogeny, which starts with fertilization of the egg and proceeds under concrete environmental conditions, is also controlled by heredity. Thus, the set of genes that the organism receives from its parents (the genotype) differs from the complex of the organism’s traits at various stages of its development (the phenotype). The role of the genotype and the environment in the formation of the phenotype may differ. But it is essential to bear in mind that the organism’s normal reaction to environmental influences is genotypically determined. Because changes in the phenotype are not adequately reflected in the genotypic structure of the sex cells, the traditional view of the inheritance of acquired characteristics has been rejected for having no factual basis and for being theoretically unsound. The mechanism by which heredity is realized in the development of an individual seems to be related to the successive action of different genes. Heredity depends on the interaction of the nucleus and cytoplasm, in which various proteins are synthesized in accordance with a program coded in DNA and transmitted to the cytoplasm by messenger RNA.
The laws of heredity are very important in agriculture and medicine. They are the basis for developing new plant varieties and animal breeds and for improving existing ones. The study of the laws of heredity led to the scientific substantiation of already known empirical breeding methods and resulted in the development of new methods (experimental mutagenesis, heterosis, and polyploidy, for example). Discoveries in human genetics showed that the genes responsible for the development of various anomalies and hereditary metabolic, mental, and other diseases are fairly common. Genetic counseling is designed to help reduce the probability of the birth of children with hereditary diseases. Early diagnosis permits the prompt institution of the required therapy. Heredity is an important factor in the reactions of people to drugs and other chemical substances and in human immunological responses. There is no question that molecular genetic mechanisms play a role in the etiology of malignant tumors.
The phenomena of heredity appear in different forms, depending on the level of life at which they are studied (molecule, cell, organism, or population). Ultimately, however, heredity is maintained by the replication of the material units of heredity (genes and cytoplasmic elements) whose molecular structure is known. The regular, matric character of their replication may be disturbed by mutations of particular genes or by reconstructions of the genetic systems as a whole. Any change in a replicating element is invariably inherited.
P. F. ROKITSKII