Genomic imprinting is a normal but complex genetic phenomenon, that is difficult to define. As of March, 2003, no adequate explanation has been found for why genomic imprinting exists.
With genomic imprinting only one type of gene (allele) is expressed while the other allele remains genetically silent. Which allele is expressed and which remains silent depends on from which parent the genes are inherited.
A small subset of approximately 50 genes exhibit characteristics of genomic imprinting.
Following fertilization of a mammalian embryo most of the genes contributed by each parent begin to function equally. When a gene is expressed, the copy inherited from the mother (maternal allele), and the copy inherited from the father (paternal allele), are transcribed equally (bi-alleleic expression) and the RNA is translated into the protein product.
In contrast, with imprinting one allele is transcribed while the other is silent (i.e., imprinted). For example, in humans the insulin-like growth factor 2 gene (IGF2), which is an important fetal growth factor, is only expressed from the paternally inherited allele while the maternal allele is imprinted and never normally expressed. Similarly, the H19 gene, which is located adjacent to IGF2, is normally only expressed from the maternally inherited allele, while the paternal allele is silent.
The genetic mechanism of genomic imprinting remains uncertain but research indicates that some form of reversible genetic modification (epigenetic modification) such as DNA methylation is involved.