WIKIMEDIA, JANE GITSCHIERMary Lyon, “one of the foremost geneticists of the 20th century” according to a statement by the Medical Research Council (MRC) Harwell, died last month (December 25). She was 89.

Lyon graduated from the University of Cambridge in 1946, a time when women did not officially receive degrees from the institution. She went on to pursue a PhD with R.A. Fisher in Cambridge and Conrad Hal Waddington in Edinburgh, completing her thesis in 1950. She then continued as a postdoc in Waddington’s lab, studying the mutagenic effects of radiation in mice, before moving to her own facility at MRC Harwell in 1955. Lyon headed up the genetics section there from 1962 to 1987.

In 1961, she developed the idea of X-chromosome inactivation, the random switching off of one of the two X chromosomes in each cell of female animals, from her studies of X-linked mutations in the mouse...