Father's or Mother's gene. Which one will be expressed?
It has been long thought that (As in
Mendelian genetics) parental origin of gene does not affect its
expression. With respect to autosomal genes, male and female
contribute equal number of genes and produce similar effects.
But expression of some genes is
significantly affected by the parental type, i.e. whether it came
from male or female. This phenomenon is called Genomic Imprinting.
One of the example of genomic
imprinting is Igf2 (Insulin like growth factor II). It is present in
both mice and humans and in both cases genomic imprinting takes
place.
A child inherit one allele from male
parent and other allele from female parent. The paternal copy of Igf2
is actively expressed in the fetus and placenta but maternal copy
remains completely silent.
So, when the paternal copy is deleted
in the mice, the mice produces small placenta and low weight baby.
Another example would be Prader-Willi
and Angelman syndromes. A child with Prader-Willi syndrome are
mentally retarded, has small hands and feed and poor sexual
development. Angelman syndrome has completely different symptoms like
frequent laughter, uncontrolled muscle movement and a large mouth.
Both the syndrome is caused due to deletion at the same region on
long arm of chromosome 15. The only difference is if the chromosome
with deletion is inherited from father, will result in Prader-Willi
syndrome were as if the chromosome with deletion is inherited from
mother, will result in Angelman syndrome.
The exact mechanism of genomic
imprinting is still under investigation, but different methylation of
DNA is one of the reasons behind it.
Genomic imprinting is one form of
epigenetics, where process like acetylation of histone proteins, or
phosphorylation and methylation of histone proteins or methylation of
DNA itself result in a different phenotype. For example, even though
homozygotic twins have same genotype, they differ in many ways
because of epigenetics.
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