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Course: AP®︎/College Biology>Unit 7

Lesson 4: Hardy-Weinberg equilibrium

Applying the Hardy-Weinberg equation

The Hardy-Weinberg equation can help to estimate allele frequencies in a population. Dominant (p) and recessive (q) allele frequencies and genotype frequencies can be calculated using the equation p² + 2pq + q² = 1. In this video, eye color is used as an example to determine allele frequencies and genotype distribution. Created by Sal Khan.

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• some people have blue eyes when they are born and then gradually with time their eyes keep changing colour. How is this possible?
• Great question! Babies eye color changes when the pigment producing cells aren't active at birth. Over time these cells start to produce pigment and the eye color of the baby changes.
• So are genetic diseases like cloud blindness or hemophilia can be fixed.....? Can you knock out the genes with those mutations in a person and replace with other genes?
• yes they can be cured by gene therapy.the defective gene is removed and a normal gene is replaced there.but at present practically gene therapy is still in infant stage
• I'm a little confused, In the other genetics class I'm taking, they said that Hardy Weinberg is used to calculate the expected genotype frequency, which is then compared to the actual genotype frequency and used for things like determining gene flow. But here, it seems like Hardy Weinberg is being used to calculate the actual genotype frequency? I thought that the criteria for Hardy Weinberg could never actually be met, and one of them was infinite population size, not just a large population as stated in this video?
• Yes Hardy-Weinberg is mainly used to calculate the expected frequency assuming: no mutations, no gene transfer, random mating, large population, and no selection. However if we know the actual frequency of the homozygotes (i.e. p^2 and q^2) in the actual population we can compare to an expected value. So p+q should = 1 , in a real population if p^2 = .36 (p=0.6) then q would have to be 0.4. If it deviates from that value then the system isn't in H-W equilibrium. So by comparing the actual to the expected you can determine if a population is in equilibrium or if they are changing (evolving).
• If a person has different colored eyes, does that increase the chance that the offspring will have two different colored eyes?
• Yes, it's true that animals like what Dhruv Indiresh mentioned might have different coloured eyes. But this is also possible in humans. It's called Hererochromia, when a person has 2 different coloured eyes. There are several people with such a case. The list is here http://en.wikipedia.org/wiki/List_of_people_with_heterochromia.
I hope this helped and I'm sure you might be able to do more research on how this occurs in Humans now :)
Edit: Heterochromia Iridum, to be exact.
• how should I know which hardy Weinberg​ equation should use?
• I know that this is a late response, but for anyone else who has this question, the p+q=1 equation is used to find the allele frequencies themselves, whereas the p2+2pq+q2=1 equation is to find the number of individuals, or the population, that has that specific genotype, or set of alleles.
• In these videos we have taken the theoretical case of an eye colour gene which has only two variants and thus the Hardy-Weinberg Equation applies with such ease as in multiplication of binomials and such
But in the real world we have numerous different genes with numerous different allele possibilities so how does the Hardy-Weinberg equation apply on such large and complex scales??
• "how does the Hardy-Weinberg equation apply on such large and complex scales??"

In short, it doesn't. There is no population where you can observe the occurrence of a particular phenotype, and then determine exactly the proportion of individual alleles.

The conditions for a Hardy-Weinberg equilibrium are pretty strict, but there are some populations that will exhibit some of these conditions a bit more than others. So in some cases, it can be used as a rudimentary predictive tool, but should not be solely relied upon.
• When sal was getting 2pq, how did he end up with 0.42?
• There are two ways of solving for this - in the video Sal plugged in the probability of "p" and "q" which were 0.7 and 0.3 respectively into the equation. He also could have subtracted the values of p^2 and q^2 from 1. Either way, the end result comes out to 2pq = 0.42.
• My professor told our class that we are not supposed to sqrt the q^2 term, since that does not give us the true q value, which he said must be solved by including the q in the heterozygotes.

I'm slightly confused...
• In this case q is recessive, so if someone has blue eyes, they will always have the genotype bb. If we were talking about people with brown eyes, we would have to include the 2pq term because people with brown eyes can be either BB or Bb, but a heterozygote will always have brown eyes, so we cannot include heterozygotes when we count the probability of an individual having blue eyes.