Mary Frances Lyon FRS[5][6] (15 May 1925 – 25 December 2014)[3] was an English geneticist best known for her discovery of X-chromosome inactivation, an important biological phenomenon.

She went on to do her PhD with Ronald Fisher, who was Professor of Genetics in Cambridge, where she characterised a mutant mice strain with a 'pallid' mutation and published the research.

The group was funded by the Medical Research Council, and she worked with TC Carter to investigate mutagenesis and the genetic risks of radiation.

In 1994 she won the Mauro Baschirotto Award in Human Genetics, in 1997 the Wolf Prize for Medicine, for her hypothesis concerning the random inactivation of X-chromosomes in mammals.

Her nomination for the Royal Society reads: Distinguished for many important contributions to mammalian genetics, notably on the mutagenic effects of irradiation as measured in mice, and on the building of the hypothesis that one of the two X chromosomes of the female is inactivated at an early stage of embryogenesis.

The hypothesis, which is now almost universally accepted as proved, offered a solution to the long standing problem of X-dosage compensation in the female mammal, has thrown much light on the nature of sex-chromosome aneuploidy, has influenced ideas on the origin of certain tumours and of chronic granulocytic leukaemia in man, and has provided food for thought about the fundamental mechanism of switching off genes.

[6]In 2018, the International Mammalian Genome Society established the Mary Lyon Award in recognition her role as a mentor and her remarkable career which began in a time period where very few women became scientists.