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MOSAICISM (synonym mosaicity) — simultaneous manifestation at the same individual of two or more alternative (is normal mutually excluding) signs. The individual possessing such signs is called to mosaics. E.g., at the person cases of mosaic (spotty) pigmentation of hair, eyes, cases of existence at the same face of a kariotipicheska of various somatic cells or different genetic options of separate proteins etc.

M. are known is called genetic when the corresponding cells differ not only on the shown signs, but also on the to a genotype (see). The main reasons causing a genetic raznokachestvennost of cells of one individual are somatic mutations (see) — gene, chromosomal, genomic, cytoplasmatic — and a somatic crossing-over (see. Recombination ), occurring in separate cells and originative hereditarily to the changed clones, i.e. posterity of these cells.

As the number of cells of a body at metaphytes is very big, despite a relative rarity of separate vegetative mutations and a somatic crossing-over, any individual is to some extent mosaic. The m can be received artificially by means of mutagen impact on cells of the developing organism or by merging of the cells taken from different, the embryos genotypic differing. In this case such artificially received mozaik more often call chimeras (see), or allofenny organisms.

Distinguish M. from genetic M. phenotypical, at Krom take place only of distinction in manifestation of any signs, but not in genotypes of the corresponding cells. Phenotypical M. on the signs linked to a gender of women and females of mammals is especially widely known (effect Lyon): in an early embryogenesis of female individuals one of X-chromosomes in kernels of somatic cells is inactivated (see. Sex chromatin ), at the same time regarding cells X-chromosome of a maternal origin, and in a part — X-chromosome of a fatherly origin, as a rule, differing with the allelic structure at least as regards genes is inactivated, as causes phenotypical M. of a female body in lack of genetic distinctions between the cells composing it.

To phenotypical (at mammals) or genetic (at insects) M. belong cases of development in one individual of generative organs and signs of both floors, so-called. gynandromorphism (see). Such genetic phenomenon as M., widely use for studying of genetic and cellular bases gisto-and an organogenesis.

Bibliography: Lobashev M. E. Genetics, page 206, etc., L., 1967; Stern K. Fundamentals of genetics of the person, page 191, etc., M., 1965.

V. I. Ivanov.