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Human Genetics for the Social Sciences
Lyonization
Here are some pictures of Peanut, a calico cat who provides an excellent example of Lyonization. Calicos have three different colors of fur--white, black, and orangish brown. The areas of white fur are due to the spotting gene that determines whether or pigment pigment is present. In the white area, there is no pigment. In the black and orange areas, pigment is present.
The difference between the orange and the black areas is determined by Lyonization. All calicos are female and have a pigmentation gene on the X chromosome. One X chromosome contains an allele that results in black pigment while the other X has a different allele that gives orange pigment. Where there is black fur, the X chromosome with the black allele is active while the orange X is a Barr body. Just the opposite occurs in the orange areas--the X with the organge allele is active while the X with the black allele is deactivated. Notice the mosaic pattern of the cat's fur. If the cells of her liver or any other organ were "color-coded" to illustrate which X was active, the organ would have a similar mosaic appearance.
The mosaicism is suspected to be the reason why female heterozygotes do not suffer from X-linked recessive traits even though the X chromosome with the "good" allele is inactivated in 50% of her cells in an organ. In the other 50% of the cells, the good allele is expressed and for reasons that are not entirely clear, that 50% is enough to keep an organ functioning.
