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Transcript
Epigenetics: The study of the mechanisms by which genes bring about their phenotypic effects In most cases the maternal and paternal genes (alleles) in an individual turn on and off at the same time Exceptions have been found where imprinting occurs causing only one of the alleles, either the maternal or paternal to be expressed and the other silenced Big-bottomed sheep – an example of epigenetics What is happening? - the callipyge trait (plump rump) is due to a single mutation on chromosome 18 - this mutation affects the expression of several genes making them hyperactive (over-express) - two such genes involve: - increased protein synthesis - increased active RNA synthesis (gene or genes that only code for RNA) - imprinting - increased protein synthesis – paternal expression - increased RNA – maternal expression So if mutation comes from the male you get big bottomed sheep If the mutation comes from the female you get normal sheep If you have an individual that is homozygous for the mutation the mutation is silenced resulting in svelte sheep Silencing could be due to the two genes interfering with each other, or the level of expression of the two is too high in the cells, triggering silencing Interestingly this is a functional example of over-dominance where the heterozygote will show greater expression than either homozygote Process involved in imprinting is not well understood but may relate to methylation. Epigenetics and imprinting may explain differences (discordance) observed for gene expression in identical twins For some traits its been shown that there a difference in imprinting can lead to a difference in expression in twins Reference: Scientific American Dec. 2003 pages 108 – 113
