Hardy Weinberg Equilibrium

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.

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? Can anyone confirm if this is correct?

Edited January 19, 201213 yr by raid517