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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