Influence of genome imprinting on gene expression, phenotypic variations and development.

Genome imprinting confers functional differences on parental chromosomes as a result of the differences in epigenetic inheritance from parental germlines. Repressed and derepressed chromatin structures probably constitute the initial germline-dependent 'imprints'. Any subsequent modifications, such as DNA methylation, will be influenced by these initial epigenetic modifications. Hence, epigenetic modifications of parental alleles probably occur progressively and this will affect their potential for expression. It appears that imprinting of some parental alleles is critical for their dosage, affecting embryonic growth, cell proliferation and differentiation. Genetic studies highlight the influence of subsets of imprinted genes and identify those which are crucial for development. Genomic imprinting also affects some transgene loci and dominant mutations with accompanying variable penetrance and expressivity. The response of transgenes can be influenced by modifier genes whose presence is most readily detected in different inbred backgrounds. The influence of modifier genes can in turn be affected by their parental origin, perhaps partly by the maternally inherited oocyte cytoplasmic factors, as well as by complex interactions between some parental alleles and oocyte cytoplasmic factors. The resulting epigenetic modifications of unlinked loci can result in substantial phenotypic variations.