Mosaic
Each multicellular organism is the result of repeated divisions of the original fertilized egg. Before each division, a cell's DNA is copied so that each new cell receives a complete copy of its genome. Copying errors (mutations) occasionally happen. If an error goes uncorrected, then all its progeny cells will carry the change, as well, and will be genetically different from the original cell. The resulting organism is called a genetic mosaic. Patches of skin different in color from surrounding skin are an example of a mosaic effect.
Another cause of mosaicism is the X-chromosome inactivation that occurs in all female mammals. In humans, for example, X inactivation is thought to happen at or before the 32-cell stage. In this process, one of the X chromosomes in each cell is randomly inactivated. For a given cell, all its progeny cells have the same X chromosome inactivated. Therefore, a female mammal is in effect a genetic mosaic because some of its cells express the alleles on the maternal X chromosome, and some the alleles on the paternal X chromosome. A visually striking example of this phenomenon occurs in the calico cat. The X chromosome of the cat contains a coat-color gene that has two alleles - orange and black. If the cat is heterozygous for this gene, then some of its cells and their progeny will express the orange allele and others the black allele, resulting in the familiar colored patches of fur.
It is also possible to be mosaic for number of chromosomes. Occasionally during mitosis, a chromosome is lost from one daughter cell, causing it to have one chromosome too few. One of the more common examples of this is the loss of the Y chromosome from a cell in a male, resulting in a person having some cells that have the normal 46, XY complement and others that have the 45,X (Turner syndrome) complement. Another example is the loss of an X chromsome from a cell of a normal XX female, resulting in a mosaic containing 45,X and 46,XX cells.
In all cases of mosaicism, the phenotypic effect on the organism is determined by the location of the changed cells and by their number, which is in turn dependent upon how early in development the change occurred. There is one extreme condition in Drosophila known as bilateral gynandromorph, in which an X chromosome is lost from one cell during the first mitotic division. The half of the fly missing an X chromosome expresses the recessive characteristics, white eye and miniature wing; whereas the other half, which is heterozygous for both genes, expresses the dominant characteristics, red eye and normal wing.
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