Mosaicism

Mosaic (genetics)
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In genetics, a mosaic, or mosaicism describes the presence of two or more populations of cells with different genotypes in one individual, who has developed from a single fertilized egg.^[1] Mosaicism has been reported to be present in as high as 70% of cleavage stage embryos and 90% of -stage embryos derived from in vitro fertilization.^[2]

Genetic mosaicism can result from many different mechanisms including chromosome non-disjunction, anaphase lag and endoreplication.^[2] Anaphase lagging is the most common way by which mosaicism arises in the preimplantation embryo.^[2] Mosaicism can also result from a mutation in one cell during development in which the mutation is passed on to only its daughter cells. Therefore, the mutation is only going to be present in a fraction of the adult cells.^[1]

Genetic mosaics may often be confused with chimerism, in which two or more genotypes arise in one individual similarly to mosaicism. However, the two genotypes arise from the fusion of more than one fertilized zygote in the early stages of embryonic development, rather than from a mutation.

Different types of mosaicism exist, such as gonadal mosaicism (restricted to the gametes) or tissue or somatic mosaicism.

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