Female mammals have two X chromosomes and one of these in every cell is inactivated. The inactivation occurs early in development when the embryo consists of a few cells. The inactivation is random – one X chromosome may be inactivated in one cell and the other X chromosome inactivated in another cell. Once a chromosome is inactivated it remains inactivated in all descendent cells.
With respect to their X chromosomes, female mammals are chimeras. A chimera is an organism composed of different genotypes. Some areas of their bodies have one X chromosome turned off and other areas have the other X chromosome inactivated.
An example of X-linked chimeras that can be seen visually are female cats that are heterozygous at the orange coat color locus. Orange is a locus on the X chromosome. Females with one allele for orange color and one for non-orange (tortoise shell and calico cats) have blotches of orange and other colors all over their bodies. Each blotch of color is composed of cells descended from a single cell in the embryo at the time of X-inactivation.
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