For some genes, only one copy is expressed. Expression of these genes is variable depending on which parent the gene came from, a process known as genomic imprinting. In this way, the active gene is preferentially transmitted from one parent over the other.
Imprinting does not occur on every chromosome; only nine chromosomes are known to have regions of genes that are imprinted.
Imprinting occurs by a pattern of methylation, meaning the copy of the gene to be inactivated is coated with methyl groups. This takes place before fertilization, in the egg and sperm cells. The methylation prevents that gene from being expressed.
Genomic imprinting is a reversible form of gene inactivation and is not considered a mutation. For instance, Jane inherits two copies of a paternally imprinted gene. The copy she inherited from her father will be imprinted, or inactivated, and the copy she inherited from her mother will be active (not imprinted). However, all the imprinted genes that Jane passes onto her offspring must be maternally imprinted. Therefore, a conversion of imprints occurs in each individual’s egg and sperm cells during their embryonic development, so that the genes they pass on reflect their gender, and not their parents’.