Mosaicism is the phenomenon where different cell populations within the same individual (having developed from the same fertilised egg) have different genotypes. Mosaicism typically results from a mutation or chromosomal aberration that occurs only in one cell lineage during development. It comes in two flavours: gonadal mosaicism, which affects some gametes while others remain normal, and tissue mosaicism, which affects some tissues while others remain normal.

In individuals who have trisomies, mosaicism may occur where non-disjunction in a mitotic division during development causes certain lineages of cells to have the usual disomy restored. This leads to a milder phenotype in trisomic disorders compared with non-mosaic individuals. For instance, in males with XXY syndrome (Kleinfelter's), where there is mosaicism, some cells have the genotype XXY and others the usual XY, resulting in less severe symptoms. In very rare cases, mosaicism can lead to an intersex phenotype, where some cells carry the female XX genotype and others the male XY genotype.

Mosaicism should not be confused with X-inactivation, where different X chromosomes are expressed in different cells of female mammals. X-inactivation is an epigenetic variation between cells and not a genotypic one.