X Chromosome & Female Development
A normal female mammal has two X chromosomes while a normal male mammal only has one. Furthermore, the Y chromosome is very small and has relatively few genes. Since we have already seen that a change in the amount of chromosomal material is generally fatal in mammals, this dichotomy causes a dilemma. How do males and females both survive if they have different amounts of X genes?
In 1962, Mary Lyon proposed the answer to this question. She used Murray Barr's observation of darkly staining bodies in female, but not male, nuclei, along with her own observations of coat color inheritance in cats, to propose that the 'extra' X gets inactivated in female mammals. She further proposed that this inactive X was the darkly staining body that Barr saw. Later experiments confirmed this hypothesis by showing that XXY males have one "Barr Body," while XXX females have two. In other words, males and females each have a single active X chromosome, and any extras are inactivated to become the Barr Bodies that you can see in this animation.
In mammals, X-inactivation is a random process that occurs early in development. Either the maternal or paternal X chromosome is equally likely to be inactivated in a nucleus. Once the choice has been made, however, all progeny of that cell will inactivate the same X.
This random inactivation can be directly observed in cats. The gene that determines whether the cat will be orange or black resides on the X chromosome. Females that are heterozygous for both alleles display a mottled black and orange pattern called tortoiseshell. Each black patch represents a group of cells that inactivated the X chromosome containing the orange allele. Conversely, each orange patch represents a group of cells that inactivated the X with the black allele.