Animal Genetics

a branch of genetics that studies heredity and variation chiefly in farm animals but also in domestic and wild animals. It is based on general genetic principles and concepts, and it mainly uses the hybrid, cytological, population, ontogenetic, mathematical-statistical, and twin methods of general genetics.

Animals have mainly independent inheritance of characters because they have a large number of chromosomes. For example, the diploid number of chromosomes is 80 in ducks, 78 in dogs and chickens, 66 in horses, 60 in cattle and goats, 54 in sheep, 44 in rabbits, 40 in swine, 38 in foxes, and 30 in mink. The principal method of studying the inheritance of characters is hybrid analysis; this helps to determine the nature of the inheritance of many morphological, physiological, and biochemical traits, which frequently depend only on one of several pairs of genes.

Considerable attention is devoted to the biochemical properties of animal milk and blood, specifically to immunogenetics, the results of which are used to check on the pedigree of purebred animals, determine their exact origins in disputed cases, and so forth. By studying the genes responsible for biochemical properties, it is possible to analyze the structure of breeds—their lines and species—and to judge the degree of their uniformity. Research is continuing on the correlation between these genes, on the one hand, and productivity, fertility, and viability of animals, on the other.

The morphological defects and underdevelopment of individual organs in animals have been explained in genetic terms. Many developmental defects (such as a bulldog appearance, dwarfishness, and dropsy in calves, rabbits, and other animals) are known to be determined by so-called lethal and semilethal genes. The individuals bearing these genes either die or have low viability. Their appearance is due to the fact that the herds contain individuals that are outwardly normal and completely viable but are heterozygous for the genes responsible for these defects. When such heterozygotes are crossed, nonviable forms appear in the offspring that are homozygous for the lethal or semilethal genes.

Genes responsible for economically useful characters may also be lethal or semilethal. A classical example of this is the dominant gene that determines the gray color, known as shirazi, of karakul lambs. It also happens to be recessive with respect to the viability of the individuals.

A new and promising area in the field of animal genetics is the genetics of resistance to infectious, parasitic, and fungous diseases. For example, there are known to be genetically determined differences in the resistance of animals to mastitis, tuberculosis, foot-and-mouth disease, and piroplasmosis.

Hereditary metabolic diseases of animals have been studied little, although by analogy with human genetics, it is possible to assume that they too are numerous.

The development of quantitative traits in animals—for example speed of maturation, milk production, fat content of milk, amount of wool clip, and egg production—depends on the activity of many of the body’s systems. This accounts for the complex genetic nature of the characters of these traits. It has been found that quantitative traits are determined by the action of many genes combined into a single action. These genes may differ in degree of dominance and include super-dominant genes, which cause heterosis in the first generation of hybrids. The methods of mathematics and statistics are used to study quantitative characters.

Breeds of farm animals and the subgroups within breeds (lines, families, and so forth) are invariably populations in which there is segregation of many of the genes. The population method is useful in studying the distribution of individual genes in animal populations. In very simple cases when there is segregation of one or more genes, the frequencies of occurrence of individual genes serve as parameters characterizing populations. The frequencies of individual genes cannot be determined by analyzing traits that depend on many genes. In this case, the coefficient of hereditability is used—the ratio of genotypic variation of a quantitative character to its total phenotypic variation. The coefficients of hereditability (from 0 to 1) vary with the specific nature of the characters for which they have been established, with the degree of uniformity of the conditions of maintenance and feeding, and with the methods by which the animals are bred. The coefficient of hereditability is useful in finding the most suitable methods of breeding and forecasting the results.