While we’re here, why don’t we take a quick look at how Hardy-Weinberg works out
with X-linked diseases? We’re all familiar with color blindness as being X-linked.
So, I’ll ask the question, what is the frequency of female carriers? Now, I’ll give you a little hint
that 8% of the population has red-green color blindness. Take a moment. See what you can do with that.
Pause the video. Take out some paper. See what you can do to calculate the frequency
of female carriers based on the fact that 8% of the population is color blind. Let’s see how you did.
There’s a little bit of a trick when we consider X-linked traits. We can do it the normal way
but a quick shortcut is looking at allelic frequency because we can look at it directly by looking at
the male because the male is hemizygous. He only has one copy of X. By looking at how many males
in the population or looking at the frequency of color blind males in the population,
we can establish q very quickly. I know, kind of sneaky, huh? But either a way, I bet you came out
with the right numbers. If we look at q and we say it’s 0.08 or 8% then we can easily,
with some simple math, figure out that p is the remainder of that, 0.92. So, 92% of the alleles
in the whole population are normal alleles. Now to the question what are we actually looking for?
We’re looking for female carriers. Female carriers are going to be heterozygous. So, they are 2pq.
All we need to do then is calculate 2pq, 2 x 0.08 x 0.92, you get the picture. We find out that
actually color blindness is carried by 15% of women. That sounds like a pretty high frequency, doesn’t it?
Either way, it’s not important that, oh, maybe it is that you remember that 15% of women carry
the color blind allele. Probably it will stick anyway. The more important thing is that you know
how to make these predictions based on the allelic frequencies and genotypic frequencies
observed in the population. Again, spend some time practicing with this stuff. If you've become
really, really familiar with it, no amount of stress in your exam can throw you off.
You just know how to work with the equation.