Each somatic cell of the human body contains 46 chromosomes consisting of two sets of 23; one inherited from each parent. These chromosomes can be categorized as 22 pairs of autosomes and two sex chromosomes. Similarly at the molecular level, two copies of each autosomal gene exist, one copy derived from each parent.
Until the mid-1980s, it was assumed that each copy of an autosome or gene was functionally equivalent, irrespective of which parent it was derived from. However it is now clear from classical experiments in mice and from examples of human genetic diseases that this is not the case.
The functional activity of some genes or chromosomal regions is unequal, and dependent on whether they have been inherited maternally or paternally. This phenomenon is termed ‘GENOMIC IMPRINTING’ and the activity or silence of an imprinted gene or chromosomal region is set during gametogenesis.
Genomic Imprinting involving the autosomes appears to be restricted to eutherian mammals. When normal pattern of imprinting is disrupted, the phenotypes observed in humans and mice are generally associated with abnormal fetal growth, development and behaviour, illustrating its importance for a normal intrauterine environment.
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