Mosaicism

Humans have twenty-three pairs of chromosomes, including one pair of sex chromosomes and twenty-two pairs of autosomes. The sex chromosomes are either X or Y chromosomes. Females have two X chromosomes, and males have an X and a Y chromosome.

In mammals, the sex of an individual is generally determined by whether the individual inherited an X or a Y chromosome from the father. The Y chromosome contains the SRY (Sex-determining Region Y) gene that directs male sexual development, but holds relatively few other genes. Many of the several dozen genes or gene families on the Y chromosome are necessary for the production of sperm. A handful are shared with the X chromosome, which is a medium-sized chromosome that is likely to contain more than one thousand genes.

Lyon knew that female mice that had only a single sex chromosome, the X chromosome, were normal. 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.