Hardy Weinberg Equilibrium?

[jebus197]

Hi in the following pages from a popular text book why in box 23.6 does it tell you that the population/sample size (in this case 200, or simply N), must be doubled in the equation to give 2N, to give p= 180+40/400, where 400 is equal to 2x N? In other words why do we need to double the population size to get the answer? I'm fine with the rest of the equation, so it's just this bit I'm stumped on.

It might help if whoever replies to this reads both pages before responding to get an idea of what I'm driving at. This section is somewhat pre-Hardy Weinberg, but I will get around to that in due course once I have got the hang of this.

[canalon]

It took me 15s of reading to find the explanation in the textbook, since it is in the first column of the first page... You might want to read your text again.

[jebus197]

Yeah I know you're bright. But I just wasn't getting it, that's kind of the point. I read it several times. So as far as I understand it, the reason you divide the sum by 2N, is because if p= 2N(AA) + N(Aa) /2N then this means that both phenotypes are diploids? So to get the value of p (which is just the value a single allele within the population), you have to divide both sides of the equation by 2 to get the haploid value? This then allows you to ask how many copies of a single allele are present within any population?

[jebus197]

I'm sorry, but I tend to see math and go a bit blank, lol.

[JackBean]

not really. In the second paragraph you have, how to calculate p as number of this allele per sum of alleles. Since the organisms are diploid, the sum of alleles is twice the number of individuals.

[JackBean]

but it's not to get some haploid numbers

[jebus197]

Now you are just confusing me lol. If the point of dividing by 2N is to get the sum of a particular gene in a gene pool, wouldn't this by definition give you a haploid number?

Let me try to think this through ... OK, so it wouldn't be haploid because in reality, two copies/alleles of the gene would exist within each organism within the target group being studied? So in other words it wouldn't make much sense to talk about a haploid number, because haploid has a very specific biological definition and doesn't really say anything useful? The point of dividing by 2N is just as you say, to get the overall value of the distribution of a gene within a population? Haploids don't come into it, because you are still effectively studying diploid organisms?

It's a little pedantic, but worth the extra effort to get a clearer definition maybe.

[JackBean]

what's "haploid number" for you? This has nothing to do with anything haploid. Just the organism is diploid, thus the number of all genes is double of number of organisms.

[jebus197]

Yeah that's kind of what I just said, thanks. I said haploid number isn't something useful, or real, simply because they aren't haploids. What you are looking at is the simple distribution of genes within a gene pool.

[jebus197]

How do you thank people on this forum? There doesn't seem to be any voting system for comments. But thanks anyway guys. Sometimes you just need someone to nudge you in the right direction.

[JackBean]

there's no voting system. But you can suggest it in appropriate subforum

[jebus197]

Just one last point to clear up, in the text it says the following:

... The second thing to notice is that both population 1 (consisting mostly of homozygotes) and population 2 (consisting mostly of heterozygotes) have the same allele frequencies for A and a. Therefore, they have the same gene pool for this locus. However, because the alleles in the gene pool are distributed differently, the genotype frequencies of the two populations differ. Genotype frequencies are calculated as the number of individuals that have the genotype divided by the total number of individuals in the population. In population 1 in Figure 23.6, the genotype frequencies are 0.45 AA, 0.20 Aa, and 0.35 aa.

Could someone please give me some examples of what might cause the distribution of these frequencies to differ? Why don't they always remain fixed in a strict mathematical relationship? I mean the relationship is still mathematical, but what could cause an allele to be expressed more frequently between one group and another? This is the final thing I need to understand about this equation I think.