I am taking cell biology and have this question:
During the process of gene expression, it is possible to express either the maternal allele or the paternal allele.
When and how is the determination made to express one allele over the other?
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Except in the case of X inactivation in females, and genes on the Y chromosome in males, generally speaking both alleles are expressed in cells.
A good example is the ABO blood type marker system. If the mother is type AA, she will pass on an A allele, which codes for the A antigen. If the father is type BB, he will pass on a B allele, coding for the B antigen. Therefore, any children will be type AB, as the maternal chromosome 9 will express the A form, and the paternal chromosome 9 will express B.
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$\begingroup$ I see.. thank you. But, when does this happens? I assume that two different mRNA's for each pair of homological chromosomes are leaving the nucleolus? So, when does the merging happen? $\endgroup$ – user135172 Jan 1 '16 at 8:11
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1$\begingroup$ @user135172 Yes, two different mRNA sequences will leave the nucleus after transcription. They don't "merge", each one is translated separately. In the case of my ABO example above, one set of mRNAs is translated to the A antigen, and the other set to the B antigen, and both are expressed on the cell surface (I'm simplifying the exact process tremendously, please read the wikipedia link if you'd like to know more). $\endgroup$ – MattDMo Jan 1 '16 at 21:12
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Answer is yes. The term you are look for is parental imprinting http://mcb.berkeley.edu/courses/mcb142/lecture%20topics/Amacher/LECTURE_13_Imprinting_F08.pdf
Take the insulin-like growth factor 2 (Igf2) on chr7 (autosome). This gene is imprinted such that only the parental copy is active. So if a mouse inherits a defective copy from its father, it will be stunted. However if it inherited a defective copy from its mother, it will be normal.
Last I looked (about 2 years ago) there were over 1000 genes in humans that are paternally imprinted. It also should be noted that the imprinting may only be present at certain stages in development. It is even possible that one parent's allele to be active in one development stage (father in fetus) but the other at a different stage (mother, rest of life).
This is a very exciting field. How the more complicated switching between imprinted allele occurs is still understudy. However for simply imprinting... it is caused by DNA methylation and DNA silencing by heterochromatin.
Female mammals have an additional system for differential allele expression...ChrX inactivation. In a tissue, one ChrX is randomly inactivated by heterochromatisation. So most of the gene on one ChrX are off. So some cells in a tissue will express the maternal allele, while others the parental allele....
Xchromosomes in females only? $\endgroup$ – Remi.b Dec 30 '15 at 19:26