Genomic imprinting

Genomic imprinting is a normal form of gene regulation that causes a subset of mammalian genes to be expressed from one of the two parental chromosomes. Some imprinted genes are expressed from the maternally inherited chromosomes and others from the paternally inherited chromosomes. This means that the maternal and paternal genomes are not functionally equivalent and is the reason why both a maternal and a paternal genome are required for normal mammalian development. Genomic imprinting has also been described in plants where the process is believed to have evolved independently from that in mammals, although aspects of the mechanism of imprinting may be the same in both organisms. The existence of imprinted genes adds another dimension to the patterns of inheritance predicted by Mendelian genetics. For example, imprinting disorders have been described. These exhibit parental origin effects in their patterns of inheritance. In these disorders, males and females are usually equally affected, however the defect is manifest only upon inheritance from a parent of one sex. Inheritance from the parent of the opposite sex does not result in an abnormality because the defective gene may be repressed on the chromosome derived from that parent (Figure 1).