In 1961 Mary Lyon, an English scientist, hypothesized that one of the two X chromosomes in females becomes genetically silent early in a female embryo's development. To understand how she arrived at this idea, which has come to be known as "the Lyon Hypothesis," we need to understand what was known about the sex chromosomes.

The Sex Chromosomes

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...