Having trouble with applying the Hardy-Weinberg Principle

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jaddison
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Having trouble with applying the Hardy-Weinberg Principle

Post by jaddison » Sun Sep 16, 2007 7:01 pm

Here's the question I'm working on:

5. In this month's issue of the Journal of Irreproducible Results, researchers from the National Institute of Hair (NIH) report that they have identified a single genetic locus that codes for hair style. There are two alleles at this locus (denoted "P" and "p"), and the system is codominant. The genotypes and associated phenotypes are as follows:


Genotype
Phenotypic Characteristics

PP
"Pompadour" -- wavy hair

Pp
"Permed" - tightly curled hair

pp
"Punk" - spiked hair

Collecting data on a random sample of 2,000 individuals at locations throughout North America, the researchers observed the following breakdown of genotypes:

PP - 1,000
Pp - 800
pp - 200

a) What are the current allele frequencies for the alleles at this locus? Is the system in Hardy-Weinberg Equilibrium?


And here are the results I came up with:

p = Fr(P) = [2(1000) + 800]/4000 = .7
q = Fr(p) = [2(200) +800]/4000 = .3

This system is not in H-W equilibrium because the expected genotype frequencies (based on the allele frequencies) for PP, Pp, and pp (.49, .42, and .09, respectively) do not match the observed frequencies for PP, Pp, and pp (.5, .4, and .1, respectively).

How can it work that the expected genotype frequencies don't match the observed ones when the allele frequencies that gave me the expected genotypes (using p x p, 2pq, and q x q) came from the observed genotypes! I'm new to all of this Hardy-Weinberg business, so I'm probably missing some crucial bit of information. Does codominance alter the results? It seems like it should, but I can't see how it would since it would only affect phenotypes (giving 3 possible phenotypes instead of 2), and we aren't dealing with phenotypes in this problem. Thanks much for anyone that can help clear this up!

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MrMistery
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Post by MrMistery » Sun Sep 16, 2007 7:36 pm

it's like this:
in the given situation, you calculated the allele frequencies from the observed genotypes. OK. however, what decides whether the population is in HW equilibrium is not the frequencies of the alleles, but the frequencies of the genotypes. and those don't match. that's why it's like that.
This deviation can have many causes. Imagine the following situation: you have the frequencies .7 and .3. But they are totally separated, and you only have homozygous individuals. those with PP don't like to mate with pp and vice-versa. now, although your frequencies are p+q=1 you won't have the HW proportions cause the population is not pancmictical(mating is not random)
Hope it is clear now
"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

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