Mary Lyon

Mary Lyon was an English geneticist who discovered X-chromosome inactivation, the process by which one of the two X chromosomes in each cell of a female mammal is randomly silenced early in embryonic development, a phenomenon now often called Lyonization in her honor. Working at the Medical Research Council radiobiology unit at Harwell in 1961 while studying the effects of radiation on mouse mutations, Lyon noticed that a coat-color mutation called mottled produced a distinctive patchy pattern in female mice but not males, and through careful breeding experiments she showed the mutation was carried on the X chromosome and concluded that one X chromosome must be switched off at random in each cell of the developing female embryo. Her hypothesis explained why female mammals, despite carrying two copies of every X-linked gene against a male one, do not typically produce twice the dosage of X chromosome gene products, and it also explained why female carriers of X-linked genetic disorders often show mild or patchy symptoms rather than full-blown disease. Lyon studied zoology at Cambridge under Ronald Fisher, completed her doctorate at the University of Edinburgh in 1950, and headed the Genetics Section at the Harwell radiobiology unit from 1962 to 1987. She received the Royal Medal in 1984 and the Wolf Prize in Medicine in 1997.