Mary Lyon's research in mouse genetics has contributed to the understanding of human genetic inheritance. Her discovery of X-chromosome inactivation, the process whereby one of the two X chromosomes in females becomes inactive early in embryonic development, furthered the understanding of the types of diseases that arise from X chromosomal genetic mutations. She has been responsible for advances in the field of environmental mutagenesis and has pioneered novel ways of assessing the risks posed from chemical mutagens.

Mary Lyon was born in England in 1925. She studied at Cambridge University, where she received a B.A. degree in 1946, and her Ph.D. in 1950. From Cambridge she moved to the Medical Research Council Radiobiological Unit in Harwell, England, where she remains. From 1962 to 1986, Lyon headed the unit.

In 1962, Lyon proposed the Lyon Hypothesis (now considered as the Lyon Principle) to explain why the phenotypic effect of the X chromosome is...