Gametic imprinting at the mouse and human IGF2R/MPR300 locus

Abstract

Mammals inherit a maternal and a paternal homologue of every autosomal gene. Remarkably, a small number of autosomal loci are not equally expressed from the maternal and paternal allele. Instead, a phenomenon known as gametic (or genomic) imprinting, acts to restrict expression to one parental allele. Since the possession of two functional gene copies confers an obvious advantage in protecting the organism from genetic mistakes, the existence of imprinted genes has stirred considerable interest. The most burning question of ‘why’ gametic imprinting exists in mammals is difficult to address directly. However, supplementary questions, such as the type of gene that is imprinted and the molecular mechanisms involved in allele-specific expression, can be directly investigated. For the purposes of this chapter, gametic imprinting is defined as ‘a reversible process whereby a gamete-imposed modification in the parental generation can lead to functional differences between the maternal and paternal allele in the offspring’ (Barlow, 1994). Imprinting is thus a phenomenon that can be traced back to differential activities of the parental gametes which result in autosomal genes showing parent-of-origin effects.