JannickL OP t1_ix5abu2 wrote on November 20, 2022 at 9:21 PM

Reply to comment by aTacoParty in How does the body know which allele is dominant and which is recessive? by JannickL

May I ask in the case of Hemophilia A, why isnt the recessive gene getting expressed alongside of the dominant? If the dominant is outperforming the recessive why does the recessive gene have zero impact instead of a lesser one. In the case of eye colour I can understand that higher melanin -> darker eye colour, so even if you have blue and brown genes you end up with brown eyes just maybe little bit lighter due to there only being one brown eyes gene. Is it the same with Hemophilia A or is there another reason why the recessive gene has no impact? Is it because one gene alone can encode for enough proteins so that overproduction in the case of the gene being there two times gets mitigated by some other factor?

aTacoParty t1_ix5ftfp wrote on November 20, 2022 at 9:59 PM

Great question. This is a special case since the hemophilia A gene is only on the X chromosome and not on the Y chromosome. Since XY males will only have one copy of the X chromosome, they only need to inherit one mutant allele to develop disease. The Y chromosome doesn't have a copy of that gene to make up for the mutant one. In XX females, they have two copies of the gene and thus will need two copies of the recessive disease allele to develop disease (just like genes on any other chromosome).

All chromosomes except sex chromosomes are symmetrical so this is a special case that makes XY males more susceptible to certain genetic diseases.