Figure 1. Chromosomal regions in the mouse with imprinted genes. The overt phenotypic effects of the imprinted regions are indicated. Some genes recently identified are outside the genetically defined imprinted regions.

Figure 2. A cluster of imprinted genes on the mouse distal chromosome 7. The domain consists predominantly of genes which display expression of the maternal allele. Evidence suggests that expression of Igf2 and H19 is linked mechanistically while other genes further upstream are regulated independently of the two genes. The figure also illustrates the key methylation difference on parental chromosomes.

Figure 3. The key events associated with genomic reprogramming in the germline. The parental imprints are first erased in the male and female primordial germ cells as judged by demethylation of these loci. The homologous chromosomes thus attain an identical epigenetic state probably for the only time in the life of a mammal. The new imprints are germline-specific and they are initiated during gametogenesis. This process is essentially complete by the time sperm and oocytes are formed. After fertilization, the new imprints are propagated during development and they can persist into adulthood. As a result, only one parental allele is expressed, resulting in the monoallelic expression of imprinted genes in appropriate cells.