Dictionary of Human Evolution and Biology

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Mosaicism, Chromosomal

1. most commonly refers to a somatic mutation during development, so that an individual is a mosaic, e.g. Down syndrome in only some cell lines.

2. normal females are mosaics for the expressed/inactivated X chromosome, with some cell lines expressing the mother’s and some the father’s X-linked alleles.

3. another (very rare) form of mosaicism occurs when two embryos fuse very early in development and express, in one organism, the genotypes of two individuals.

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