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in each cell of an individual heterozygous at an autosomal locus, the nonpreferential suppression of the phenotypic manifestation of one or other of the alleles; the phenotype of the body is thus mosaic. Compare: lyonization.
allelic exclusionA phenomenon in which one of the two genes for which an individual is heterozygous is expressed, while the other gene is excluded (i.e., not expressed, or inactivated). Allelic exclusion is typical of the expression of the Ig superfamily of genes, and is characteristic of B and T cells. Allelic exclusion explains a cell’s ability to express only one of the two possible allelic forms of Ig or T-cell receptor.
Per the Lyon hypothesis, one of the X chromosomes of female mammals is inactivated early in development, and individual cells express only one allelic form of the product of that locus; because X chromosome inactivation is a random event, different cells express one or other of the X chromosome products, resulting in mosaicism.