Mendel’s first law implies the equivalence of reciprocal crosses, i.e. allelic effects are independent of their parental origin. Since the 1980’s, however, approximately forty genes have been identified that are subject to parental imprinting in eutharian mammals (e.g. Reik and Walter, 2001). Paternal and maternal alleles of imprinted genes are differentially methylated during their passage in the respective germ line resulting in their functional nonequivalence. Typically, one of the alleles – the paternal allele for approximately half the imprinted genes, the maternal allele for the other half – is transcriptionally silent in at least some tissues. Imprinted genes are typically clustered in chromosomal domains containing both paternally and maternally expressed genes that may be under coordinate control of common “imprinting centers” (ICs). Imprinted domains are characterized by a number of idiosyncrasies including the frequent occurrence of non-coding as well as anti-sense RNAs, a high incidence of CpG islands, the presence of tandem repeats, a paucity of SINE sequences, allelic replication asynchrony, and a higher male versus female recombination rate. 

M. Georges, Francis Barillet, N. Buys, C. Charlier, C. Colette, E. Davis, F. di Silvestro, L. Moreau, C. Nezer, M. Nguyen, K. Segers, T. Shay, M. Smit, N. Cockett

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 2002. Session 15, , 15.04, 2002
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