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What does the concept of "extreme heterozygosity" mean?

I first encountered this concept in "The Drunken Botanist". They describe that when planting a seed from, say, a 'red delicious' which was pollinated by another 'red delicious' does not guarantee growing another 'red delicious' tree. The author goes so far to say that the new tree may have almost none of its parent traits, and may barely resemble any other breed whatsoever. They argue this is caused by extreme heterozygosity.

I later found the concept of extreme heterozygosity the Wiki page for "apple"

This is because seedling apples are an example of "extreme heterozygotes", in that rather than inheriting DNA from their parents to create a new apple with those characteristics, they are instead significantly different from their parents.

On a PNAs article on Extreme genomic variation referencing extreme heterozygosity

We conclude that the extreme heterozygosity in C. savignyi is caused by a large effective population size and not an elevated mutation rate.

My understanding of population genetics (allele frequencies and patterns of dominance)

I understand homozygous diploid pairs, like pp or PP, and heterozygous pairs like pP and Pp, and hetero/homozygous dominance and recession. I also found an interesting paper on sea squirts to work through later that suggests why extreme heterozygousity is popular in large population sizes. What I can't seem to find is exactly how "extreme heterozygousity" works. How is this different than ordinary heterozygous? Is there an entirely different context I'm missing out on? The only way I can imagine a more "extreme" heterozygosity is with triploid mismatched pairs, which can't be right. So how does the process work and what exactly defines it?

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  • $\begingroup$ Can you please link to your article? I have never heard of "extreme heterozygosity" and it is pretty hard to say anything without seeing the paper. $\endgroup$
    – Remi.b
    Aug 28, 2015 at 4:23
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    $\begingroup$ Large population size tend to have higher heterozygosity and there is less drift to wipe out genetic variance. I guess this is what your paper is refering to given that you say extreme heterozygousity is popular in large population sizes. The term popular seems a little inappropriate though. You should cite correctly. $\endgroup$
    – Remi.b
    Aug 28, 2015 at 4:26
  • $\begingroup$ I am voting to close as you're asking to comment on a claim and a concept without providing the claim or the context in which you found this concept. Of course, I'll remove my vote close as soon as you link to the article. $\endgroup$
    – Remi.b
    Aug 28, 2015 at 4:27
  • $\begingroup$ Please take a look at my edits. $\endgroup$
    – chauxvive
    Sep 2, 2015 at 16:23
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    $\begingroup$ Thank you! That is much better with the links. I am surprised that even in the PNAs paper, they don't seem to offer any good working definition. I voted to reopen. I also made important edits to your post. Feel free to roll back or make any further modifications. $\endgroup$
    – Remi.b
    Sep 2, 2015 at 18:27

3 Answers 3

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@yingw response was right, but I will try to explain it a bit better.

Say that you had a gene, let's call it the plumb gene. This presents two alleles: P and p. Those alleles behave in a way that gives three different phenotypes depending on the genotype: PP, Pp and pp all have different phenotypes, for example, yellow, green and blue color.

If the P and p frequencies are each 0.5 on the total gene pool of the population you would expect a Hardey-Weimberg equilibrium if the reproduction and survival was random (there would be no selection).

  • $0.5^2=0.25$ of homozygous (PP and pp are each a 1/4 of the total population)
  • $2*0.5^2=0.5$ of heterozygous (Pp would be 1/2 of the population)

For what you say, extreme heterozygous would mean that the rate of heterozygosity in the population would be much higher. For example, if we found 90% of green and only 5% of blue and 5% of yellow, that would mean that there is a selective advantadge for the green individuals.

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    $\begingroup$ It is also my intuition that "extreme heterozygosity" would refer to any heterozygosity higher than $0.5$. That'd be great if you'd have a reference for this definition! $\endgroup$
    – Remi.b
    Sep 2, 2015 at 18:17
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    $\begingroup$ From the article the OP linked: extreme heterozygosity in C. savignyi is caused by a large effective population size and not an elevated mutation rate.. It doesn't seem to match your definition as large population size and (reasonably) high mutation rate won't cause a heterozygosity that is greater than 0.5. $\endgroup$
    – Remi.b
    Sep 2, 2015 at 18:29
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Without reading the article, given the context that you have mentioned, I think extreme heterozygousity means that most of the individuals in the population are heterozygous (much higher than 50%). So there might be some kind of selection pressure that gives heterozygous individuals a reproductive benefit.

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  • $\begingroup$ Great insight- So the likelihood of homozygous pairs would be highly unlikely? But in diploid pairs, would that mean that rather than two options like r and R, there would be more alternatives, like say r, R, d, and D for one spot? so you would be less likely to get rr, and more likely to get rD or something? The article isn't relevant to my question. I had heard of extreme heterozygousity in apples and couldn't find any other explanations of exactly what that meant in Punett square vocab. $\endgroup$
    – chauxvive
    Aug 19, 2015 at 2:19
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    $\begingroup$ I think you might be confusing a couple of things. r and R would be two different alleles. Alleles can often be thought of as different mutations (ex. R represents the original A allele, r represents an A->T mutation, D represents an A->C mutation. Its possible that the A base pair is dominant to T and codominant with C). If you find that R allele appears in 0.5 fraction of individuals, r in 0.2 and D in 0.3 then if population is in equilibrium you expect RD = (2* 0.5 * 0.3), RR = (0.5 * 0.5), Rr (2* 0.5 * 0.2), etc. If you observe fewer RR/rr/DD then there is some selection for heterozygous $\endgroup$
    – yingw
    Aug 27, 2015 at 23:01
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According the this article: Extreme genomic variation in a natural population

There is increasing evidence that many animal species exhibit high levels of heterozygosity (1–4), which, according to the Neutral Theory, should be caused by elevated mutation rates or large effective population sizes (5).

This would imply in addition to the fact that even in a single genetically homogeneous orchard the offsprings are still very heterozygotous, that apple trees have a high mutation rate from the parents to the children.

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