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I've run into a question that I disagree with others on but I would like to receive the input of others. The question is:

If a population is at Hardy-Weinberg equilibrium, which of the following can be predicted for the recessive alleles in the population? 1) They will eventually disappear, 2) They will be selected against, 3) They will be maintained at the same frequency, 4) They will be expressed in the heterozygous condition, and 5) They will become dominant.

Some people said that the answer to this question was that the recessive alleles would be selected against, but I interpreted the question as having the answer that the allele frequency of the recessive alleles would not change and instead would be maintained at the same frequency.

My rationale for this was that Hardy-Weinberg equilibrium means there is random mating and basically no radical shifts in allele frequencies from outside sources, such as gene flow, natural selection, etc. I would think that the allele frequency would not change, but I may be incorrect.

Is my methodology correct?

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  • $\begingroup$ In short: No, you're wrong. Hardy-Weinberg is nothing but a formula that link genotypes frequencies to allele frequencies (under random mating). Allele frequencies will change in response to selection so does the genotype frequencies. The only think that doesn't change is the relationship between allele frequency and genotype frequency. (the above assumes that selection occurs on survival, not on fecundity). $\endgroup$ – Remi.b Apr 29 '15 at 3:15
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    $\begingroup$ @Remi.b I think you're conflating Hardy-Weinberg frequencies with Hardy-Weinberg equilibrum. The former are exactly what you say, "nothing but a formula that link genotypes frequencies to allele frequencies (under random mating)". The latter imply stable maintenance of these frequencies via the term equilibrium. $\endgroup$ – Corvus Apr 29 '15 at 3:56
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Let's go through the possible answers. Which of the following could be predicted? Not (1), because we don't know whether selection favors or disfavors the recessive alleles. Not (2) for the same reason. Not (4) by definition. Not (5) because recessive alleles don't magically become dominant.

This leaves us with (3), which also seems correct: not only do we know that frequencies don't change under the Hardy-Weinberg assumptions, but also an equilibrium is by definition a state at which frequencies remain the same.

An aside: this question fairly clearly recalls G. Udny Yule's confusion about the fate of recessive alleles, which lead Punnett to enlist G. H. Hardy's help in deriving Hardy-Weinberg equilibrium. Yule expected recessive alleles to decline in frequency by virtue of their recessiveness; Hardy showed otherwise.

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