Inbreeding

inbreeding,

mating of closely related organisms. Inbreeding is chiefly used as a means of insuring the preservation of specific desired traits among the offspring of purebred animals (see breeding). Continued inbreeding through many generations reduces the chances for diversity of characteristics in the offspring and tends to reduce vigor and fertility. Only in laboratory conditions can the unwanted characteristics that frequently result from inbreeding be selected out of the strain and selection for purely advantageous traits be carried out. The necessarily uncontrolled cases of inbreeding among humans (as in closed societies or within royal families) have generally proved deleterious, and inbreeding is therefore discouraged in most societies (see consanguinity; incest).

Inbreeding

 

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.

REFERENCES

Darwin, 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

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Inbreeding

 

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.

inbreeding

[′in‚brēd·iŋ]

(genetics)

Reproduction behavior between closely related individuals; self-fertilization, as in some plants, is the most extreme form.