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Question

People who are carriers for Blooms Syndrome have developed a selective advantage. How would this affect the allele frequency of the disease allele?

My thoughts

Natural selection will occur, only the carrier is selected and the affected will be selected against, leading to an imbalance of the Hardy-Weinburg equilibrium.

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    $\begingroup$ At Biology.stackexchange we expect that people show some effort to find an answer by themselves, otherwise questions are considered off-topic and will be closed. The same happens to homework/assigment questions with no effort to solve them. So, what have you found out? $\endgroup$ – Chris Aug 24 '17 at 7:11
  • $\begingroup$ i found a few different things. Natural selection will occur, only the carrier is selected and the affected will be selected against. Leading to an imbalance of the hardy-weinburg equilibrium. - sorry, it is my first time asking qns and even coming to this page so i dont really know the 'rules' of this website. $\endgroup$ – melly Aug 24 '17 at 7:43
  • $\begingroup$ Welcome to Biology.SE! I edited your post (title, tag and I added your comment onto it), to make it on-topic. Feel free to roll back if you don't like the edit. $\endgroup$ – Remi.b Aug 24 '17 at 18:07
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    $\begingroup$ This is slightly tangential, but I question one of the premises for your question ("People who are carriers for Blooms Syndrome have developed a selective advantage"). On Wikipedia <en.wikipedia.org/wiki/Bloom_syndrome> and briefly on Pubmed, I don't see any such evidence. Or was this meant to be a hypothetical for the purpose of understanding population genetics? $\endgroup$ – Ben Bolker Nov 6 '17 at 1:30
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Question

People who are carriers for Blooms Syndrome have developed a selective advantage. How would this affect the allele frequency of the disease allele?

Your answer

Natural selection will occur [..]

This is correct. Anytime you have a fitness differential among different genotypes, selection will affect allele frequencies.

[..] [l]eading to an imbalance of the hardy-weinburg equilibrium.

Hard-Weinberg equilibrium describes the relationships between allele frequencies and genotype frequencies. Selection can sure affect this relationship at some stage of life but it will not affect this relationship just after fecundation.

In any case, the question does not seem to expect you to talk about that. The question is asking about allele frequency only.

If you want to understand Hardy-Weinberg, you should definitely have a look at Solving Hardy Weinberg problems.

What I would answer

Not assuming any extra knowledge, the only simple, logic answer is to say that the frequency of the allele coding for the Blooms Syndrome will likely increase in frequency in the population, eventually reaching fixation. This increase in allele frequency would yield to an increase in the frequency of carriers of Blooms Syndrome.

Of course, if there is heterozygote advantage (aka. overdominance for fitness), then in an infinite population, the allele should never fully reach fixation.

If we were to know more about the genetics of the Blooms Syndrom or the specific selection advantage (and eventually many other things one may want to consider) one could formulate a more quantitative answer.

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  • $\begingroup$ Won't reach fixation if homozygote Bloom syndrome genotype is deleterious (as seems to be true) and only carriers have a fitness advantage (cf. sickle-cell allele) $\endgroup$ – Ben Bolker Nov 6 '17 at 1:32
  • $\begingroup$ Is there heterozygote advantage (a.k.a overdominance for fitness) in the locus causing variation for Blooms syndrom? I edited the answer adding a "if there is overdominance" type of paragraph. Thanks $\endgroup$ – Remi.b Nov 6 '17 at 22:24
  • $\begingroup$ Original question might need to be closed with "unclear what you're asking". If OP is referring to Bloom syndrome en.wikipedia.org/wiki/Bloom_syndrome , it seems pretty clear that being homozygous for the variant allele is deleterious. Thus I was guessing that the question was a hypothetical "suppose there was overdominance at this locus, what would happen"? $\endgroup$ – Ben Bolker Nov 6 '17 at 22:59

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