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The coexistence of two or more genetically distinct cell populations derived originally from a single zygote. Mosaics may arise at any stage of development, from the two-cell stage onward, or in any tissue which actively proliferates thereafter. The phenomenon is commonly observed in many species of animals and plants and may be caused by somatic mutation or chromosomal nondisjunction. An individual animal or plant may exhibit mosaicism, or it may occur in a culture of a single cell- or tissue-type obtained from an individual.
Chromosome nondisjunction is probably the principal cause of chromosomal aberration, which in turn may lead to the development of mosaicism. During cell division, the two sister chromatids usually separate completely, each chromatid going to opposite poles of the cell guided by the spindle apparatus. In some cases, one chromatid will fail to completely separate, or it may lag behind. This nondisjunction will lead to the presence of both sister chromatids in the same daughter cell instead of one in each of the daughter cells (Fig. 1). Somatic crossing-over leads to the production of a recombinant mosaic where chromosome segments, with their corresponding blocks of genes, are exchanged between homologous chromosomes during mitosis. The occurrence of this process leads to mosaicism, mostly manifested as spots (clones of variant cells) on the cuticle of insects or on leaves, petals, or stamen hairs. See Chromosome
Sex chromosome mosaicism, the presence of a mixture of cell populations with different X and Y chromosome constitutions, is not uncommon and is often seen in individuals with ovarian dysgenesis. The presence of a significant proportion of chromosomally abnormal cells in any such mosaic will tend to lead to a clinically expressed syndrome. The proportion of each constituent clone may vary from tissue to tissue, but is relatively stable in each individual site throughout adult life. Sex mosaics (gynandromorphs) are particularly striking where a difference in the secondary sexual characteristics exists between the normal sexes. For example, in a butterfly with bilateral gynandromorphism, the left side may show the characteristic wing color and pattern of the male, and the light wing, typical female patterning (Fig. 2). See Chimera, Genetics, Sex-linked inheritance
the simultaneous presence in an organism of two or more varieties of homotypic cells differing in genetic structure, or genotype, and (or) in their manifestation of genes in the phenotype. Thus, in mosaicism, characters that are usually mutually exclusive (sex characters, color, biochemical features) appear simultaneously. Examples of mosaicism are bilateral or anterior-posterior mosaic insects or mosaic birds, mottled leaves in plants, red and white facets in the compound eyes of insects, the presence of XX and XY cells in the same individual and other anomalies of certain chromosomal diseases of humans.
Mosaicism results from the following conditions: (1) improper distribution of chromosomes in mitosis, (2) somatic mutations of gene and chromosome-type reconstructions, (3) somatic crossing-over, (4) redistribution of cytoplasmic genetic factors (for example, of plastids in plants) in such a way that they are lost in part of the cells, or (5) the functioning in different cells only of some one or other of the homologous chromosomes or of one of the allelic genes. One differentiates mosaicism based on changes in the genotype (conditions 1, 2, and 3) or plasmotype (condition 4). Phenotypic mosaicism is due to a change in the function of the gene or its dose (condition 5).
V. L. RYZHKOV