Inbreeding

INBREEDING (English inbreeding, from in- in, within and breed-to breed, German Inzucht), related crossing, breeding by means of related crossings, "breeding within," "intra-marriage." Inbreeding in the broad sense is called related crossing in general; in the narrow sense (usually with the addition of words: strict, close inbreeding) - and reproduction by means of self-fertilization or by crossing brother with sister in each generation. Significance of inbreeding in the laboratory, breeding and nature. Close inbreeding occurs as a normal phenomenon in nature, for example in self-pollinating plants. Close inbreeding is a very fruitful method of breeding in genetics and breeding: with its help, pure and purebred lines and cultures of plants and animals are developed, and an experimental analysis of hereditary traits is carried out. In morphology and physiology, in the experimental study of varying traits and reactions of the organism to certain external influences, it is also necessary to resort to inbreeding to obtain as much as possible "pure," hereditarily homogeneous material, which eliminates the "obscuring" influence of the hereditary variability of this trait or reaction on the experiment. Inbreeding in the broad sense, in the form of more or less distant or close related crossings, occurs in nature in small isolated populations; in these cases, by accelerating the manifestation of the reserve of recessive genes of this population, inbreeding participates in the mechanism of the evolutionary process. The question of "degeneration" as a result of inbreeding. In medicine, animal husbandry and plant breeding, the question of inbreeding is also raised in a completely different plane. Since ancient times, the opinion has been established that related crossing, especially close and repeated, leads to the "degeneration" of the corresponding family, breed, culture or line, to the appearance of deformities in it and the weakening of its "vitality" and fertility. At the end of the 19th century, in connection with the study of the question of the potential immortality of protists (unicellular organisms), the theory of sex and Weismann's theory of amphimixis, this opinion seems to receive confirmation from general biology. At present, with the development of experimental genetics, the question of "degeneration as a result of related crossing" can be considered clarified. There is no doubt that inbreeding as such has no harmful effect on offspring. This is shown by cases of natural, inbreeding (e.g. in self-pollinating plants) and a number of precise laboratory experiments (e.g. the fly Drosophila can be bred in close inbreeding for a hundred generations without signs of "degeneration" appearing). The general biological prerequisites in favor of the old opinion have also fallen away: it turned out that in protists amphimixis can be replaced by automixis and that the "rejuvenating" effect on unicellular and some multicellular organisms, in addition to amphimixis, can also be exerted by repeated regeneration after artificial injuries.- The significance and effect of inbreeding thus comes down to the following: if in a heterogeneous environment (population), containing a number of genes in a heterozygous state, inbreeding occurs, then for a number of recessive genes the possibility of manifestation is created, passing into a homozygous state; as a result of this, a number of recessive hereditary traits appear in the offspring, which were previously in a "hidden," heterozygous state; among them, along with neutral and positive ones, pathological, negative traits (deformities, hereditary diseases, genes causing partial or complete infertility, etc.) will appear, and these latter, especially striking the eye, create the impression of the harmful effect of inbreeding. By increasing homozygosity (hereditary homogeneity) in the offspring, inbreeding thereby reduces the unrealized "hereditary potential" (of a given breed, family, line) and can, with the participation of selection, create onesidedness and specialization of the breed (for example, breeds and varieties of cultivated plants and animals).- Inbreeding in humans-see Consanguinity.

H. Timofeev-Resovsky.