Genomic imprinting affects several dozen mammalian genes and results in the expression of those genes from only one of the two parental chromosomes (Reik and Walter, 2001). These mammalian genes function differently depending on whether they came from the mother or the father (Susani et al., 1984; McGrath and Solter, 1984). A key molecular mechanism of imprinting is DNA methylation. These epigenetic instructions, called imprints, are laid down in the parental germ cells. Methylation marks the imprinted genes differently in egg and sperm, and inheritance of these epigenetic marks is known to lead to differential gene expression (Reik et al., 1987: Li et al., 1993). All the cells in a resulting child will have the same set of imprinted genes from both its father and its mother, except for the cells destined to make gametes (germplasm).
Around 45 imprinted genes have been identified in the mouse, with the majority of the genes displaying the imprinting status in humans also (Reik and Walter, 2001). A characteristic feature of imprinted genes is that they are found always accompanied by other imprinted genes: around 80% are physically linked in clusters with other imprinted genes. This organization of imprinted genes is believed to reflect coordinated regulation of the genes in a chromosomal domain, similar to X-chromosome inactivation in which an X-inactivation centre controls the inactivation of the entire chromosome. Imprinting centres or imprinting control elements (ICs) have been discovered in some clusters, and are found to be essential in controlling of imprinting or imprinted expression within distinct regions (Reik and Walter, 2001).
In addition to there spatial organization, the DNA sequence environment of imprinted genes seems to have two general features. First, they are unusually rich in CpG islands: 88% in imprinted genes versus the average figure of 47% (Paulsen et al., 2000). Second, clustered, direct repeats are common within or near the CpG islands (Neumann et al., 1995).
Imprinting genes affect growth in the womb and behaviour after birth. Aberrant imprinting disturbs development and is the cause of various disease syndromes. For example, in humans the insulin-like growth factor 2 (Igf2) allele inherited from the father, paternal, is expressed. The allele from the mother is not. If both alleles should begin to be expressed in a cell, that cell may develop into a cancer referred to as Wilms’ tumor (Vu et al., 2003).