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I just began studying population genetics, and I don't understand something that is written in my lecture:

Consequences of non-random mating:

  1. More homozygote and less heterozygote individuals in the population
  2. Allele frequencies are constant (not in case of negative density dependence)
  3. Genotype frequencies change

What I don't understand is the second point. If the genotypes' frequencies change, shouldn't the frequencies of alleles change as well? Furthermore, I don't really understand either how this is supposed to be linked to a negative density-dependence case...

Any help is welcome! :)

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Out of context at least the small piece of text you cite is very poorly phrased and partially wrong.

First, you should have a look at Solving Hardy Weinberg problems. Take your time and read that post... Done? Good.

More homozygote and less heterozygote individuals in the population

This is wrong as non-assortative mating might be disassortative mating where individuals are attracted to individuals of different genotypes.

However, it is true that population structure will cause excess of homozygotes. This is called the Allee effect. The loss of heterozygosity due to population structure is equal to twice the variance in mean allele frequency among those populations. These details sounds a bit too advance for your needs though so I won't go any further.

Allele frequencies are constant (not in case of negative density dependence) Genotype frequencies change

Constant over what? Change over what? Not over time necessarily (at least not unless some other assumptions are being made).

I guess what is meant is that if in a given generation mating is not random (due to population structure for example), then you expect the same allele frequency as if it was random but not the same genotype frequency. If it is not clear why, you should probably reread the post "Solving Hardy-Weinberg problems"


Much further discussion in both english and french on @AlissaDeschamps misunderstanding can be found here.

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  • $\begingroup$ Thanks for your answer! I read all the post about Hardy Weinberg and solved all the problems, but now I still don't understand why the allele frequencies should be constant under non-random mating, and I don't understand either how re-reading the post is supposed to help since Hardy-Weinberg is assuming random mating... I imagined a situation where there would be non-random mating (such as positive assortative mating) and a change in genotype frequencies, but I got a change in allele frequencies as well... So, could you please help me a little bit more? Thanks in advance! $\endgroup$ – justdoit Jun 16 '17 at 15:31
  • $\begingroup$ I think your issue has to do with the phrasing (but I am not sure). The allele frequency is the same whether in a given generation mating was random or not. It does not mean that the allele frequency is constant over time. If mating is not random, then the H-W expectation won't hold and genotype frequencies won't be that easy to extrapolate from allele frequencies. But it does not change the allele frequency in any way different than if matin was random (assuming there is no selection at least). $\endgroup$ – Remi.b Jun 16 '17 at 15:37
  • $\begingroup$ Let me know if that helps and don't hesitate to keep going with further question to narrow down to what is unclear to you. $\endgroup$ – Remi.b Jun 16 '17 at 15:37
  • $\begingroup$ Thanks a lot for the help, again. However, I'm now more confused than before. If it does not mean that the allele frequency is constant over time, then what does "allele frequencies are constant" mean at all?... (I'm now considering trying to answer the question "what are the consequences of non-random mating" without looking at my lecture, since it became so confusing to me... but if you're ok to help me again, well, it would be welcome and greatly appreciated.) $\endgroup$ – justdoit Jun 16 '17 at 15:44
  • $\begingroup$ Totally out of subject, but I just visited your website, looked at your CV and noticed that you studied in Neuchâtel :) That's the city I come from and I'm studying biology in Fribourg... le monde est petit! ;) $\endgroup$ – justdoit Jun 16 '17 at 15:49

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