Methylation

Methylation can also function in the process of genomic imprinting, which is found in sexually reproducing species. During sexual reproduction, each parent contributes one allele for each gene to the offspring. Genomic imprinting is a difference in gene expression that depends on whether the gene allele originated from the mother or the father. This differential pattern of gene expression occurs as the result of differential methylation in the gene promoter. One example of an imprinted gene is the insulin-like growth factor II (IGF2) gene. There are two copies (or alleles) of this gene, but only one is expressed. For IGF2, the maternal allele is methylated and silent, whereas the paternal allele is unmethylated and expressed. The opposite situation may occur in other genes. In this way, only one copy of an imprinted gene is expressed, and this provides a mechanism for a cell to determine the parental origin of certain genes.