Mammalian X chromosome inactivation, an example of mitotic memory

The third lecture dealt with the inactivation of the X chromosome, a prime example of the heritability of a state of gene activity across cell divisions and developmental dynamics. In 1961, British mouse genetics specialist Mary Lyon observed that mammalian females, which carry two X chromosomes, exhibit unexpected "mosaic" phenotypes of coat color - unlike males, which carry a single X and Y chromosome. Mary Lyon proposes that these result from the random inactivation of one of the two X chromosomes in each cell of the early embryo, followed by the stable transmission of this silent state during successive cell divisions. As soon as the two X chromosomes carry different forms of the same gene, controlling coat color for example, the random inactivation of one or other of these two chromosomes and the ensuing clonal expansion of cells produces, in the adult individual, a juxtaposition of cell patches with distinct phenotypes. Advances in our understanding of the molecular mechanisms underlying the establishment and maintenance of X-inactivation, and the implications of this cellular mosaicism in females, were presented in detail.