X chromosome inactivation, also called the Lyon hypothesis, is a phenomenon that occurs in every female mammal that has the double X chromosome configuration. Based on her own work on mice with mottled coats, Mary F. Lyon of the Medical Research Council's Mammalian Genetics Unit in Harwell, England, proposed in 1960 the idea of X inactivation. To ensure that the sexes work with similar doses of X genes, which scientists believe is critical for embryonic development, female mammals evolved the ability to inactivate one of their sex chromosomes. Early in embryonic life, one of the X chromosomes in every cell randomly shuts itself down. Researchers have uncovered evidence that DNA sequences usually dismissed as junk DNA without any function actually may help determine what genes on the X chromosome are suppressed. Some of the X chromosome genes, however, resist inactivation. Huntington F. Willard of Case...