The genetic information contributed by eggs and spermatozoa to development of embryos to term has until recently been assumed to be equivalent. However, evidence now suggests that in the mouse and perhaps in all mammals the parental genomes have complementary roles during embryogenesis and both are essential for development to term (Surani, Barton & Norris, 1984; McGrath & Solter, 1984a; Cattanach & Kirk, 1985; Surani, 1986). Functional differences between the parental genomes presumably arise as a consequence of specific modifications of homologous chromosomes in the germline, when they are segregated and exposed to different epigenetic factors during oogenesis and spermatogenesis. These ‘imprinted’ modifications of parental chromosomes are then brought together following fertilization and subsequently propagated to influence events throughout development. It is also essential that at some stage of ontogeny the specific modifications are reversed and the whole process repeated in the formation of new germ cells.

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