Genetic mosaicism occurs when two or more populations of cells with their own individual genetic genotypes are present within one individual when this individual developed from a singular fertilized egg. This is not the case when the individual has developed from a merging of two or more different fertilized eggs such as chimerism. Genetic mosaicism can occur due to a variety of different reasons including anaphase lag, endoreplication and chromosome non-disjunction.

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Female calico cats are an example of genetic mosaicism because they are developed from one singular egg. In this case, the color alleles for black and orange are located along with the X chromosome. As the X chromosome is inactivated it allows for the color variations seen in calico cats. Therefore, different genotypes are seen within the same individual. The cat is developed from a single egg but as the X chromosome is inactivated it results in a variation of cat patterns.

If a mosaicism occurs only in somatic cells, only the parent will be affected by the change. Somatic cells are not passed on to children so there is no way that the child will have the same mosaicism that the parent has. The genetic mosaicism that occurred did so in the individual and will not be passed to children in genetic material.

If the mosaicism occurred effects both somatic and germ cells there is a possibility that the child will inherit the same genes from the parent and exhibit the same mosaics. Germ cells are male sperm or female egg cells that must come together and share their half genetic material to build a singular individual and because germs cells only carry half of the genetic material from one parents, it is not guaranteed that the child will inherit the mosaicism.