If females have two X chromosomes and males have only one, why doesn’t that create a biological imbalance? In 1961, British geneticist Mary Lyon proposed a simple theory of X-chromosome inactivation in females. Lyon explained that, During early embryonic development, each cell in a female embryo makes a random choice. It switches off one X chromosome, either the maternal X or the paternal X. This choice remains permanent, and all daughter cells inherit the same inactivated X chromosome throughou

X Inactivation Animation