Genomic imprinting is an epigenetic phenomenon that causes genes to be expressed in a ‘parent-of-origin’ specific manner.
In diploid organisms, like humans, the somatic cells possess two copies of the genome, one inherited from the father and one from the mother. Each autosomal gene is therefore represented by two copies, or alleles, with one copy inherited from each parent at fertilization.
For the vast majority of autosomal genes, expression occurs from both alleles simultaneously. In mammals, however, a small proportion (<1%) of genes are imprinted, meaning that gene expression occurs from only one allele. The expressed allele is dependent upon its parental origin. For example, the gene encoding insulin-like growth factor 2 (IGF-2) is only expressed from the allele inherited from the father.
Discovery of Imprinting
In the 1980’s, scientists attempted to create parthenogenetic diploids (gynogenetic or androgenetic) diploids in mammals by putting either two female pronuclei or two male pronuclei into mouse eggs and then transferring the eggs into a foster mother.
Control embryos derived from fusion of a maternally-derived pronucleus and a paternally-derived pronucleus developed normally, but embryos from the fusion of two maternally-derived pronuclei or two maternally-derived pronuclei did not develop normally.
The only possible genetic difference between males and females in this experiment was the sex chromosomes, but even XX animals failed to thrive. We know now that one of the reasons that mammalian gynogenetic and androgenetic diploids cannot be made is because of an epigenetic phenomenon called genomic imprinting. The expression of an imprinted gene depends upon the parent (maternal or paternal) that transmits it.
Epigenetic means ‘outside the genes’. Epigenetic inheritance describes a variant condition that does not involve a change in DNA sequence, yet is transmitted from one somatic cell generation to the next during development and growth of an organism.
Maternal Imprinting and Paternal Imprinting
Maternal imprinting means that the allele of a particular gene inherited from the mother is transcriptionally silent and the paternally inherited allele is active. Paternal imprinting is the opposite; the paternally-inherited allele is silenced and the maternally-inherited allele is active.
How to Identify Maternal and Paternal Imprinting in Pedigree?
- In paternal imprinting, paternally-inherited allele is inherited in silenced state, however, half the progeny of affected females will be affected irrespective of their gender.
- In maternal imprinting, maternally-inherited allele is inherited in silenced state, however, half the progeny of affected males will be affected irrespective of their gender.
Genome imprinting does not obey classical Mendelian inheritance pattern.
Hypothesis for Origin of Imprinting
A widely accepted hypothesis for the evolution of genomic imprinting is the ‘parental conflict hypothesis’. Also known as the kinship theory of genomic imprinting, this hypothesis states that the inequality between parental genomes due to imprinting is a result of the differing interests of each parent in terms of the evolutionary fitness of their genes.
The father’s genes that encode for imprinting gain greater fitness through the success of the offspring, at the expense of the mother. The mother’s evolutionary imperative is often to conserve resources for her own survival while providing sufficient nourishment to current and subsequent litters.
Accordingly, paternally expressed genes tend to be growth-promoting whereas maternally expressed genes tend to be growth-limiting.
In support of this hypothesis, genomic imprinting has been found in all placental mammals, where post-fertilisation offspring resource consumption at the expense of the mother is high; although it has also been found in oviparous birds where there is relatively little post-fertilisation resource transfer and therefore less parental conflict.