Mary Lyon discovered the process of X-inactivation, in which one X chromosome is largely silenced in each mammalian female cell.
Lyon proposed the idea in 1961 after studying coat-color patterns in mice. Female mammals generally have two X chromosomes, while males generally have one X and one Y. Silencing one X chromosome helps balance gene activity between the sexes.
The inactive X chromosome can appear as a dense structure called a Barr body. Because different cells may silence different X chromosomes, female mammals can show mosaic patterns, such as the patchwork coat of a calico cat.
X-inactivation is not always complete: some genes escape silencing. The process is also important in understanding X-linked disorders, because cells may express different copies of an X-linked gene depending on which chromosome remains active.