Animal genetics is a branch of veterinary medicine that studies heredity and variation in 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. 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. Research is continuing on the correlation between these genes, on the one hand, and productivity, fertility, and viability of animals, on the other. 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. 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 semi lethal genes. The individuals bearing these genes either die or have low viability.