A mosaic in the biological sense, also called a genetic mosaic or somatic mosaicism, refers to an individual whose cells have a different genetic makeup. Mosaics result from mutations in somatic cells after fertilization. Thus, they emerge from a homogeneous zygote (when cell populations are genetically different, they are called chimeras).
A genetic mosaic can affect all organs. If the skin is affected, such a mutation can lead to visible changes. A cutaneous mosaic is recognizable as a characteristic pattern (see Blaschko lines below) (Bae T et al. 2018).
Mosaics with chromosomal aberration are not uncommon in sex chromosomes as a result of loss of an X chromosome (e.g. Turner syndrome with karyotype 45,XO/46,XX).
As a result of X chromosome inactivation, females have a functional mosaic, with some of the maternal and some of the paternal X chromosome active in the cells. Many mosaic mutations are not detectable in the blood, but only in the affected tissue.