Genomic Imprinting in Mammals
A zygote inherits a haploid set of chromosomes from both parents. Hence, its phenotype is supposed to result from co-expression of both alleles received from the sperm and the ovum. However, in mammals, gametes do not contribute equal genetic functions to the embryo due to the phenomenon of genomic imprinting.
Genomic imprinting is a mechanism that differentially marks maternal and paternal chromosomes, resulting in differential expression of the modified genes. Thus, some genes are expressed only from maternal chromosome and some from paternal chromosome only. The process ensures requirement of both parental genomes for production of a normal offspring. Offsprings produced from a single parent (bimaternal or bipaternal) fail to complete embryogenesis. Also, if the dosage of imprinted genes is altered, it can have profound effects on embryonic development. E.g., uniparental disomy or deficiency involving imprinted genes have been implicated in several human disorders.
At molecular level, genomic imprinting is caused by DNA methylation in CpG dinucleotides and differential histone acylation. Both methylation at 5-position of cytosine and hypoacylation of histone molecules have been shown to result in silencing of the imprinted gene.
February 24th, 2009 at 6:35 am
In what way is “Genomic Imprinting” (as described above) different from “Allelic Exclusion”?
March 16th, 2009 at 3:47 am
Genomic imprinting is very different from allelic exclusion. In genomic imprining the silencing (of either paternal or matrenal allele) is predetrmined. Allelic exclusion (as occurs in expression of antibody genes), on other hand, is a random process. Here the genes are not imprinted to be expressed or silenced. The gene to be silenced is decided in the progeny. Mechanism of silencing are also different in the two cases. Please visit an earlier blog on this site (X-chromosome inactivation) for an example.