Final answer:
British geneticist Mary Lyon answered the question about the X chromosome and discovered X-chromosome inactivation in female mammals, where one of the X chromosomes is randomly inactivated during early development.
Step-by-step explanation:
British geneticist Mary Lyon answered the question about the X chromosome in relation to X-chromosome inactivation. She discovered that in female mammals, one of the two X chromosomes is randomly inactivated during early development, forming a structure called a Barr body. The inactivated X chromosome remains condensed and genes on it are not expressed. This process compensates for the double genetic dose of X chromosome in females.