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the crossing of closely related forms within a single population of organisms. The closest form is self-fertilization. Inbreeding results in increased constancy in the offspring. In inbreeding, an organism heterozygous for a given pair of genes (aA) produces offspring, half of which are heterozygous (2Aa) and the other half homozygous (1AA + 1aa). The number of heterozygotes will be (1/2)2 = 1/4 in the second hybrid generation, (1/2)3 = 1/8 in the third, and (1/2)n in the nth generation. With self-fertilization of all the heterozygous individuals of a population, in each succeeding generation half the genes that were previously heterozygous will become homozygous.
Inbreeding is a normal phenomenon in self-pollinating plants (such as wheat, barley, peas, beans, pepper, citrus fruits, and cotton). In plant crosses and animals, inbreeding may result in manifestation of the effect of harmful recessive genes, which in a homozygous state cause partial death (sublethal and subvital genes) or total death (lethal genes) of the organisms. The harmful effect of inbreeding can often be seen, for example, in self-pollinated corn, potato, and cabbage (decrease in growth rate and fecundity, development of anomalies and malformations). In chickens, annual mating of “brother × sister” reduces the egg yield and viability of the offspring. Among human beings, the incidence of many hereditary diseases, especially rare ones, is several times greater in the offspring of marriages between first cousins than it is in other children; congenital malformations, stillbirths, and death in childhood are 20–48 percent more frequent. The depression caused by the appearance after inbreeding of individuals homozygous for harmful genes is most pronounced in the first generations, after which it does not develop beyond a certain level (inbred minimum). It is sometimes possible to produce even lines possessing normal viability in animals that have been inbred for a long time.
Because inbred individuals also become homozygous for normal genes, including those responsible for economically valuable characters, they have a great capability for transmitting valuable properties to their offspring. Crossing two or more inbred lines frequently results in heterosis in the first hybrid generation, a phenomenon extensively used in plant growing and poultry raising. Corn tetrahybrids—plants produced by crossing four inbred lines—have yields that are 30–60 percent higher than those of the ordinary varieties. A promising approach in livestock raising is the use of top crosses, that is, the crossing of inbred sires with unrelated females not subjected to inbreeding. The Soviet zootechnician M. F. Ivanov regarded inbreeding as an indispensable part of the method of creating new breeds, and he used it to produce the Ukrainian white swine breed. In almost any breed of animal, homozygosity for several practically valuable characters can be induced by close relatives. This is the way the highly productive mating of the Shorthorn and Aberdeen Angus cattle breeds was achieved.
Under natural conditions, inbreeding in self-pollinating plants often alternates with cross-pollination. The former guarantees the hereditary stability of the offspring, and the latter guarantees heterosis and hereditary differentiation. In closed mountain valleys and on small islands, the natural isolation of small plant and animal populations is conducive to inbreeding and the appearance of rare recessive forms. They include, for example, the nonligulate forms of rye and soft wheat found by N. I. Vavilov in the Pamirs, nonligulate hard wheat on the island of Crete, the early-maturing form of corn, and American upland cotton in western China.
REFERENCESDarwin, C. Diestvieperekrestnogo opyleniia isamoopyleniia v rastitel’nom mire. Moscow-Leningrad, 1939.
Pisarev, V. E. “Intsukht.” In Teoreticheskie osnovy selektsii rastenii, vol. 1. Edited by N. I. Vavilov. Moscow-Leningrad, 1935.
Kislovskii, D. A. Izbr. Soch. Moscow, 1965.
Stern, C. Osnovy genetiki cheloveka. Moscow, 1965. (Translated from English.)
Borisenko, E. Ia. Razvendenie sel’skokhoziaistvennykh zhivotnykh, 4th ed. Moscow, 1967.
IU. P. LAPTEV
the final stage of pedigree crossbreeding, that is, the discontinuation of further outbreeding and the mating of two related crossbreeds, if they meet the established requirements.