Mary F. Lyon
Wolf Prize Laureate in Medicine 1996/7
Mary Frances Lyon
Affiliation at the time of the award:
Medical Research Council, Mammalian Genetics Unit, England
“for her hypothesis concerning the random inactivation of X-chromosomes in mammals.”
Dr. Mary Frances Lyon put forward early in 1961 a disarmingly simple yet elegant hypothesis regarding the random inactivation of one of the two X-chromosomes carried by female mammalian cells.
Somatic cells of male mammals carry only one X-chromosome, whereas cells of females carry two. Since many of the genes encoded on the X-chromosome must be expressed to the same extent in both male and female cells, some mechanism must exist to compensate for this imbalance in X-chromosome number. The Lyon Hypothesis was put forward as a means of achieving this dosage compensation for the genes encoded on the X-chromosome. In a concise one-page paper (Nature 190: 372, 1961), Lyon argued that this balance was brought about by the random inactivation of either the paternally or maternally inherited X-chromosome in each female cell and that this inactivation occurred early in embryonic development.
The original arguments were made on the basis of genes expressed in the mouse, including coat color genes, but this hypothesis explained X-chromosome behavior in many – if not all – mammals. Over the years her hypothesis has been confirmed by much indirect and, more recently, direct evidence focusing on the gene(s) responsible for this behavior.