Genetic imprinting

Genetic imprinting is a phenomenon, which involves differences in expression of genes inherited from mother and father. In other words, the chromosomes in the sperm and egg are differentially imprinted or marked as having come from the father or the mother, so that some maternally inherited genes and other paternally inherited genes are differentially expressed in the progeny.

Any epigenetic changes in the germ cells, such as the inactivation of an X-chromosome, are reversed or altered during or just before meiosis, which leads to the production of sperms and eggs. After this reversal of epigenetic changes, the eggs and sperms are differentially imprinted. As a consequence of this, even if a chromosome in a sperm happens to have been inherited from the fathers’ mother, it would still bear the male imprint (Fig. 17.24). Such imprinting has been found to be crucial to normal development in mice and other mammals, because it silences and activates different sets of genes in the maternal and paternal chromosomes. These differences in paternal and maternal chromosomes complement each other in directing normal development. This differentiation or reprogramming of the genome is believed to be achieved through changes in methylation patterns. In mice, differential methylation of paternal and maternal chromosomes has actually been demonstrated.

A case of genetic imprinting has also been shown in human patients suffering with a disease known as Beckwith Wiedermann Syndrome (BWS), where both copies of a region on the short arm of chromosome 11 (Up 15.5)were inherited from the father. This feature is described as paternal disomy. A similar situation has been shown involving paternal disomy for mouse chromosome 7 (homologous to human chromosome 11), which led to production of abnormally large embryos, a condition analogous to BWS in humans. This is attributed to differential paternal expression (imprinting) of a gene Igf2(insulin like growth factor-2) in mouse, which is believed to be homologous to BWS-gene in humans.