What is Mosaic (genetics)?
[[File:Heterochromia.jpg|thumb|right|230px|This girl has one brown eye and one hazel/green eye.]] [[File:Calico cat - Phoebe.jpg|thumb|right|230px|A typical [[calico cat]]]] In [[genetics]], a '''mosaic''' (or mosaicism) means the presence of two different [[genotype]]s in an individual which developed from a single [[Fertilization|fertilized egg]]. As a result, the individual has two or more genetically different cell lines derived from a single [[zygote]].<ref>The term has also been used for organisms with cells derived from more than one zygote.</ref> Mosaicism may result from: #[[Crossing-over]] during [[mitosis]] #A [[gene]] [[mutation]] during [[development]] #A [[Chromosome|chromosomal]] mutation during development #[[X-inactivation]]: one [[X chromosome]] is randomly switched off in [[cell]]s of a female [[mammal]] The [[phenomenon]] was discovered by [[Curt Stern]]. In 1936, he demonstrated that [[recombination]], normal in [[meiosis]], can also take place in [[mitosis]].<ref>Stern C. 1936. Somatic crossing-over and segregation in ''Drosophila melanogaster''. Genetics '''21''', 625{{ndash}}730.</ref> When it does, it results in somatic (body) mosaics. These are organisms which contain two or more genetically distinct types of [[tissue]].<ref name=Curt>Stern, Curt 1968. Genetic mosaics in animals and man. pp27{{ndash}}129, in Stern C. ''Genetic mosaics and other essays''. Harvard University Press, Cambridge, Massachusetts. </ref> == Chimeras == People who have mosaicism are often called [[Chimaera (genetics)|chimeras]] but this is a mistake. A mosaic is originally from a single fertilised egg, whereas a chimaera comes from two fertilised eggs. == Other causes of two-tone appearances == This is easiest to see with [[eye colour]]s. When eye colours vary between the two eyes, or within one or both eyes, the condition is called ''[[heterochromia]] iridis'' (= 'different coloured iris'). It can have many different causes, both genetic and accidental. For example, [[David Bowie]] has the appearance of d