She also knew that mice carrying two different genes for coat color, one on each X chromosome, exhibited a mosaic, or blotchy, pattern of coat color. Some cells expressed one color gene, while others expressed the other, producing a mottled pattern.
Finally, she knew that when female cells are stained and looked at under a microscope a darkly staining region called a Barr body can be seen. She hypothesized that in female cells the Barr body is an inactive X chromosome. Thus, only one X chromosome would be active in any cell, resulting in a mottled pattern of X-linked gene expression. Furthermore, female cells lacking an X chromosome would be all right if the remaining X was the active one.
Mosaic Expression
A good example of an animal that exhibits mosaicism is a tortoiseshell cat, which has patches of black and orange fur. There is a dominant gene on the X chromosome that makes the cat's fur orange. If a female cat has this geneon only one of its two chromosomes, then the pigment-producing cells in which this chromosome is active will generate orange fur, while those that have the gene on the inactive X chromosome will make black fur.
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