Genetics, Human


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Genetics, Human

 

a branch of genetics closely related to anthropology and medicine. It is arbitrarily divided into two areas: anthropogenetics, which studies the heredity and variation of normal characters in man, and medical genetics, which studies hereditary pathology (diseases, defects, anomalies, and so forth). Human genetics is also related to evolutionary theory (in that it investigates the specific mechanisms of human evolution and man’s place in nature), psychology, philosophy, and sociology. Cytogenetics, biochemical genetics, immunogenetics, genetics of higher nervous activity, and physiological genetics are the most rapidly developing fields of human genetics.

Instead of the classical hybridological analysis, human genetics uses the genealogical method, which consists of analyzing the distribution of a particular character or anomaly in the families (more precisely, genealogies) of individuals who may or may not posses it in order to discover the type of inheritance, frequency, and intensity of expression of the character. Analysis of familial data also reveals the degree of empirical risk, that is, the probability of possessing a character in relation to the closeness of kinship with its carrier. The genealogical method has shown that more than 1,800 morphological, biochemical, and other kinds of human characters are inherited in accordance with Mendel’s laws. For example, dark skin and hair are dominant over light; and low level of activity or absence of certain enzymes is determined by recessive genes, whereas height, weight, intellectual capacity, and some other characters are determined by “polymeric” genes, or systems of many genes. Many sex-linked inherited human characters and diseases are caused by genes located in the X or Y chromosome. About 120 such genes are known. They include the genes of hemophilia A and B, deficiency of the enzyme glucose-6-phosphate-dehy-drogenase, and color blindness.

The twin method is also used in human genetics. Identical twins develop from a single egg fertilized by a single sperm; hence, the set of genes (genotype) in identical twins is identical. Fraternal twins develop from two or more eggs fertilized by different sperm; hence, their genotypes differ as much as those of siblings do. A comparison of the differences in pairs between identical and fraternal twins reveals the relative significance of heredity and the environment in determining the properties of the human organism. Of particular importance in research on twins is the index of concordance, which expresses (in percentages) the probability of one identical or fraternal twin having some character if the other twin possesses it. If the character is determined chiefly by hereditary factors, the percentage of concordance is much higher in identical twins than in fraternal twins. For example, concordance in blood groups, which are determined only genetically, is 100 percent in identical twins. For schizophrenia, concordance is 67 percent in identical twins and 12.1 percent in fraternal twins. In congenital feeblemindedness (oligophrenia), the figures are 94.5 and 42.6 percent, respectively. Similar comparisons have been made for a number of diseases. Thus, research on twins shows that heredity and the environment differ in their contributions to the development of a great variety of characters, and that these characters develop as a result of the interaction of the genotype and the external environment. Some characters depend mainly on the genotype, with the genotype acting as a predisposing factor in the formation of other characters (that is, a factor limiting the standard reaction of the body to an environmental action).

The human genome contains several million genes capable of influencing the development of characters in different ways. Gene mutation and recombination give rise to a wide variety of characters in man. Human genes mutate at the rate of one per 100,000 to one per 10,000,000 gametes per generation. Population genetics studies the distribution of mutations among large groups of people so that charts can be made to show the distribution of genes responsible for the development of normal characters and hereditary diseases. Of particular interest to population genetics are isolates, groups of people that are endogamous for geographic, economic, social, or religious reasons. Endogamy increases the frequency of consanguinity between married people; it makes it probable that recessive genes will become homozygous. The phenomenon is quite apparent when the isolates are few in number.

Research has proved the existence of natural selection in human populations. However, human selection has specific features; it is strongly active only in the embryonic stage. (So-called spontaneous abortions are a reflection of such selection.) Selection in human society is achieved by means of differences in forms of marriage and in fertility, that is, as a result of the interaction of social and biological factors. Mutation and selection are responsible for the great variety of some characters (polymorphism), which makes man an extraordinarily flexible and adaptive species from a biological point of view.

The extensive use of cytological methods in human genetics helped to develop cytogenetics, in which the main object investigated is the chromosome, a structure of the cell nucleus containing the genes. The chromosome set in human body, or somatic, cells was discovered in 1946 to consist of 46 chromosomes, with the female sex determined by the presence of two X chromosomes and the male sex by an X chromosome and a Y chromosome. Mature gametes (haploids) contain half the number of chromosomes. Mitosis, meiosis, and fertilization maintain the continuity and constancy of the chromosome set throughout both the series of cell generations and the generations of organisms. Disruptions of these processes may give rise to anomalies in the chromosome set and change the number and structure of the chromosomes, causing the so-called chromosomal diseases. The latter are often manifested by feeblemindness, severe congenital abnormalities, and anomalies of sex differentiation; they may also cause spontaneous abortions.

Advances in human genetics have made it possible to prevent and treat hereditary diseases. One of the effective methods of preventing them is medicogenetic consultation, with a prediction of the risk that an offspring of individuals suffering from such a disease or having relatives with the disease will develop it. Biochemical human genetics has discovered the ultimate cause (molecular mechanism) of many hereditary defects and metabolic anomalies and thereby has helped to devise rapid diagnostic methods that permit prompt and early detection and treatment of individuals suffering from many hitherto incurable hereditary diseases. Therapy generally consists of introducing into the body substances that are not produced there because of genetic defects, or using special diets that exclude substances having a toxic effect because of a hereditarily determined inability to break them down. Many genetic defects are corrected by timely surgery or training.

Practical means of maintaining human hereditary health and preserving the gene pool are made available through a plan of medicogenetic consultations. Their principal purpose is to inform those who are interested about the probable risk of a disease developing in the offspring. Another medicogenetic measure is the dissemination of genetic knowledge among the population in that it promotes a more responsible attitude toward parturition. Medicogenetic consultation abstains from recommendations aimed at compelling or encouraging parturition or marriage and limits itself solely to providing information. Actions aimed at creating ideal conditions for the manifestation of positive hereditary tendencies and at preventing injurious environmental factors from impairing human heredity are very important.

Human genetics is the natural scientific basis for the struggle against racism. It has conclusively shown that races are forms of human adaptation to specific environmental conditions (climatic and other) and that they differ from one another not in the presence of “good” or “bad” genes but in the distribution of the ordinary genes common to all races. Human genetics has shown that all races are equal in value (but not identical) from the biological standpoint and that they are all equal in their capacity for development, which is determined by sociohistoric and not genetic conditions. The existence of biological or hereditary differences between individual persons or races cannot be used as a basis for drawing conclusions of a moral, juridical, or social nature that restrict the rights of these persons or races.

REFERENCES

Neel, J., and W. Schull. Nasledstvennost’ cheloveka. Moscow, 1958. (Translated from English.)
Kanaev, I. I. Bliznetsy. Moscow-Leningrad, 1959.
Stern, C. Osnovy genetiki cheloveka. Moscow, 1965. (Translated from English.)
McKusick, V. Genetika cheloveka. Moscow, 1967. (Translated from English.)
Biologiia cheloveka. Moscow, 1968. (Translated from English.)
Efroimson, V. P. Vvedenie v meditsinskuiu genetiku, 2nd ed. Moscow, 1968.
Osnovy tsitogenetiki cheloveka. [Moscow, 1969.]
Li Ching-chun. Human Genetics. New York, 1961.

K. N. GRINBERG and A. A. PROKOF’EVA-BEL’GOVSKAIA

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