Direction Selection: Dominant, Additive, Recessive

[FadedFace]

Hi all...I am back with another population genetics question. For the record, this is for my course in university, "Principles in Evolutionary Biology", and population genetics is the topic for our first midterm. My instructor is quite poor, and has a very thick accent so it makes things difficult to understand. To top it off, my book briefly explains the things he wants us to know, yet dives very deep into mathematics, most of which we are not required to know.

 

Anyway, I was wondering if anyone could answer some questions for me, based on this figure:

 

 

Why does the allele frequency increase so quickly when the advantageous allele is dominant?

 

The best I can answer this, is that, even in the heterozygote form (which the mutation was in originally), the dominant and advantageous phenotype is expressed regardless of its heterozygote state. With the heterozygote expressing the "fittest" phenotype, the chances of forming homozygotes over generations is greatly increased, thus resulting in a rapid increase of the frequency of the advantageous, dominant allele. I understand that my explanation here is elementary at best, and possibly is only partially correct. I'd love for clarification on this matter.

 

Why does the additive advantageous allele reach fixation faster than the dominant?

 

From my understanding, additive mode of gene action in directional selection means that the heterozygotes will only have "half" of the fitness of homozygotes with the favored allele, so the allele frequency will increase slowly compared to when the favored allele is dominant, which has a greater fitness. That makes sense to me, I guess. However, I cannot seem to wrap my head around why it will reach fixation faster than compared to that of a dominant advantageous allele. I get that with a dominant advantageous, it will take a while to reach fixation because the recessive allele is masked in the heterozygotes...but what about with an additive advantageous? Maybe I don't really know what that means, exactly? If anyone could help me with this, I'd greatly appreciate it.

 

When it comes to a recessive advantageous allele, I really have no difficulties, as it is relatively obvious as to why it'd take so long for its frequency to rise.

 

I know these are probably very simple concepts to you all, but for some reason, evolution/genetics has always been difficult for me to grasp unless I get clear, concise explanations. That being said, I appreciate your help!

Edited February 23, 2012 by FadedFace

[FadedFace]

Hi, I actually found a journal that examined this, and it now makes sense! Thanks anyway.

[Arete]

Too late then! but anyway for anyone reading along:

 

a) Selection can only act on expressed traits. For an advantageous recessive allele, only homozygotes for the allele can be selected for. For a dominant allele, both homozygotes and heterozygotes are at a selective advantage. As such, a big slab of additional gene carriers are experiencing elevated reproductive success and the gene sweeps through a population faster when the allele is dominant.

 

b) When the gene is both dominant and the selective effects are additive, the situation is the same as above: hets are out-competing recessive homozygotes. But in addition dominant hets are out-competing both hets and recessive homs, so the sweep is more rapid again.

[FadedFace]

Thanks for the reply.

 

The conclusion I came to:

 

When it's a dominant allele, your heterozygotes are expressing the "most fit" phenotype, leading to a greater chance of reproductive success to, over time, eventually lead to homozygotes. Now, like you said, you have a "big slab" of gene carriers expressing this advantageous trait; the coupling, if you will, of homozygotes and heterozygotes with the advantageous allele leads to a massive increase of the gene frequency as it is successfully passed through the generations.

 

In the case of the additive allele, I am thinking (from what I've gathered) that since the heterozygote form is intermediate in fitness between the two homozygote forms, it will take quite some time for its frequency to rise until it starts to be inherited in a homozygote state. Once it hits the homozygote state, where its fitness is greatest (despite the absence of any true dominance), it can eventually out-compete the intermediate-ly fit heterozygotes, which over time will essentially wipe out the other allele from the population. With a dominant allele, it will mask potentially deleterious recessive alleles from selection in heterozygous form, so it will possibly never reach fixation.

 

Is this right?

Edited February 23, 2012 by FadedFace

[Arete]

When the dominant allele is not additive, the relative quotients of hets and dominant homs rises at the same rate. When the advantage is additive, the relative quotient of dominant het and homs both rise, but the relative quotient of dominant homs rises faster - progressing towards fixation at an elevated rate - as your graph suggests.

 

Dominant alleles can mask deleterious recessive alleles, but progression towards fixation by selection against recessive homozyogotes means that notwithstanding linkage and selection on associated traits - will occur.

[FadedFace]

When the dominant allele is not additive, the relative quotients of hets and dominant homs rises at the same rate. When the advantage is additive, the relative quotient of dominant het and homs both rise, but the relative quotient of dominant homs rises faster - progressing towards fixation at an elevated rate - as your graph suggests.

 

Dominant alleles can mask deleterious recessive alleles, but progression towards fixation by selection against recessive homozyogotes means that notwithstanding linkage and selection on associated traits - will occur.

 

Is that not what I said about additive advantage in my previous post, or am I interpreting what you mean incorrectly here? Just want to make sure I grasp this entirely.

 

Thanks!