Genomic Imprinting
According to Mendelian genetics, an allele should always have the same effect on an individual, regardless of whether it is of maternal or paternal descent. However, in recent years it has been discovered that in some cases the expression of particular genes is dependent upon whether they originated from the male or female parent's genome. The phenomenon by which the activity of a gene is determined by its parental origin is known as genomic imprinting.
Genomic imprinting is thought to occur during gametogenesis, and in many cases it has been found to be associated with the methylation (addition of a CH3) of the affected allele's DNA. This chemical alteration of the allele ultimately turns it off. As we discussed earlier, methylation is a common way to regulate gene activity in cells (see the advanced RNA chemistry section). However, imprinting is unique because only one allele, that of maternal or paternal origin, is inactivated in an individual and because this inactivation can be reversed in a sex specific manner during gametogenesis.
For example, let's take the paternally imprinted gene shown in this animation. As you can see, the female embryo will inherit an active form of this gene from her mother and an inactive, imprinted one from her father. However, since she is female, we know that she will pass on active copies of this gene to her offspring. This is because during gametogenesis, all of her paternally imprinted genes are reactivated in the gamete, while any maternally imprinted genes are turned off.
Genomic imprinting is believed to be necessary for proper development and growth regulation in the embryo and neonate. For example, if individuals inherit two complete copies of a maternal or a paternal genome, rather than one of each, the result is usually fatal. In this case, even though the individual has two copies of every gene, it lacks the proper balance of maternally and paternally imprinted alleles. Cancer can also be associated with improper imprinting. In Wilms' tumor cells, both alleles of the Igf2 gene, which is normally paternally imprinted, are active. Finally, imprinting can occasionally cause disease if the active allele is defective in some way. Examples of diseases caused in this manner are Prader-Willi and Angelman Syndromes.