Mosaicism

Mosaicism refers to the presence of two or more cell lines in an
individual that differ in chromosomal constitution or genotype,
but have been derived from a single zygote. Mosaicism may
involve whole chromosomes or single gene mutations and is a
postzygotic event that arises in a single cell. Once generated,
the genetic change is transmitted to all daughter cells at cell
division, creating a second cell line. The process can occur
during early embryonic development, or in later fetal or
postnatal life. The time at which the mosaicism develops will
determine the relative proportions of the two cell lines, and
hence the severity of the phenotype caused by the abnormal
cell line. Chimaeras have a different origin, being derived from
the fusion of two different zygotes to form a single embryo.
Chimaerism explains the rare occurrence of both XX and XY
cell lines in a single individual.