Unequal parental contributions: genomic imprinting in mammals.

Evidence for genomic imprinting, in which the expression of genes is influenced by their parental origin, is provided by pronuclear transplantation experiments, by studies of X-chromosome inactivation and monoparental disomy, and by the analysis of the expression and methylation of certain transgenes. Both patroclinous and matroclinous patterns of mammalian inheritance occur, and germ-cell and gonadal traits as well as tumor development can be affected. To understand the processes that influence genetic inheritance, genomic imprinting must be distinguished from Y-linkage and cytoplasmic and other maternal effects. Genomic imprinting may be mediated by a variety of mechanisms, including DNA methylation. Further studies addressing such aspects of imprinting as its initiation and maintenance are required to understand the molecular bases for these critical genetic events.