The Lyon hypothesis

A curious imbalance appears to occur in nature—the possession of two X chromosomes by one sex (usually the female) compared with only one X in the other sex. Thus, all genes on the X in the female are present in a double dose when compared with the male. Yet most such genes produce the same effect in both sexes. An intriguing theory to account for this equalization of expression has recently been proposed, and the investigation should enhance our comprehension of X chromosome aberrations, sex differences, and the basic mechanisms of gene action in the developing embryo and the maturing organism.