Lyon, Mary Frances (1925– )

English mouse geneticist who is best known for her theory, now known as the Lyon hypothesis, that one of the X chromosomes (female sex chromosomes) is inactivated during early embryonic development of a female mammal. She also helped establish the mouse as a valid experimental model for investigating the genetic basis of inherited diseases.

The Medical Research Council's Radiobiology Unit at Harwell had been constructed at the end of World War II to define the effects of radiation on mice as a model for understanding the consequences of nuclear fallout on human beings. Lyon realized that the mutations developed in the Radiobiology Unit's programme could provide valuable insights into fundamental problems in mammalian genetics.

Lyon is best known for her discovery of ‘transcriptional inactivation’ of an X chromosome in female cells. A normal diploid cell contains two copies of each autosome (any chromosome apart from the sex chromosomes) which means it has a double dose of each gene product on these chromosomes. A female cell contains two X chromosomes, while a male cell has one X chromosome and one Y chromosome, causing an apparent divergence in the gene contribution from the X chromosome between males and females. Lyon determined that one of the X chromosomes contracts into a dense object that is transcriptionally inactive.

Lyon was born in Norwich and graduated from Girton College, Cambridge in 1946. After completion of her PhD in 1950, she joined the Medical Research Council's Radiobiology Unit at Harwell and became head of its Genetics Division in 1962. She was elected a Fellow of the Royal Society in 1973 and made deputy director of the Radiobiology Unit in 1982.