A genetic mosaic, or mosaicism, involves the presence in one individual who has developed from a fertilised egg of two or more genetic populations of cells with different genotypes. Mosaicism has been reported to be present in as high as 70% of cleavage-stage embryos and 90% of blastocyst-stage embryos derived from in vitro fertilization.Genetic mosaicism can result from many different mechanisms including chromosome nondisjunction, anaphase lag, and endoreplication. [Source: Wikipedia 
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