Mary Lyon was born on 15 May 1925 in Norwich, England as the daughter of a civil servant and a schoolteacher. She was educated at a grammar school in Birmingham. During the second world war, she pursued her studies at Girton College at the University of Cambridge, where she read zoology, physiology, organic chemistry and biochemistry, with zoology as her main subject. During that time she became interested in embryology. She went on to do her PhD with R A Fisher, who was Professor of Genetics in Cambridge, where she characterized a mutant mice strain. During the course of her PhD she moved to Edinburgh.
After her PhD Mary Lyon joined the group of Conrad Hal Waddington, funded by the Medical Research Council to investigate mutagenesis and the genetic risks of radiation. In 1955, her group moved to the MRC radiobiology unit in Harwell. She has published many papers on radiation and chemical mutagenesis and on studies of mutant genes. She was head of the Genetics Section of the MRC Radiology Unit at Harwell from 1962 to 1987. She retired from active research in 1990.
It was while working on radiation hazards in 1961 that she discovered X-chromosome inactivation, for which she is best known. Her research has allowed us to understand the genetic control mechanisms of chromosome X, which explains the absence of symptoms in numerous healthy women that are carriers of diseases associated with this chromosome. An example is Duchenne muscular dystrophy. She has also done extensive work on the mouse t-complex, and made many other contributions to mammalian genetics.