Sex specific X chromosome expression caused by genomic imprinting

The conflict theory of genomic imprinting predicts that imprinted genes are growth enhancing when paternally expressed and growth suppressing when mat ernally expressed. The expression pattern of autosomal imprinted genes gene rally fits these predictions. However, the conflict theory cannot easily ac count for the pattern of X-linked imprinting in humans and mice. This has l ed us to propose a novel hypothesis that X-linked imprinting has evolved to control sex specific gene expression in early embryos. The hypothesis link s paternal X-imprinting (i.e. paternal copy silencing) to random X-inactiva tion and the retention of Y-linked copies, and links maternal X-imprinting to escape from random X-inactivation and the loss of Y-linked copies. The h ypothesis offers a good explanation of the existing data on X-imprinted gen es. (C) 1999 Academic Press.