Okay. This problem can be a little tricky if you're
not used to figuring questions like this out, backwards. But
this is the best way to learn. Instead of giving
you all the numbers and have you plug them into equations,
it's better to have you work backwards like this. 'because
you really get to see the patterns that emerge. So
let's take the easy part first. We know that if
we have no lactase non-persistent, no lactose intolerant people, zero, 0%.
That represents our homozygous recessive condition. So
we gotta have zero in that group. There
are no people in our population that have that genotype. Okay, so now that means
we gotta split 1000 people between these
two genotypes, homozygous dominant and heterozygous. The only
scenario I can calculate that gives me this
particular allelic frequency is 500 in each group.
So remember both of these are lactase
persistent, so together that's everybody 100% are
lactase persistence. But we need a number
to get our allelic frequencies to 0.75 for
the dominant allele. And 0.25 for the recessive allele, so if we go to our
equation here and set it up toward backwards. 0.75 equals, we have the total
number of alleles, so how many total
dominant alleles are we dealing with here? This
would be X equals 1500. We've got to have 1500 dominant alleles in here. We do
the same thing with the recessive allele, we gotta have 500. Well, if we have to
have 500 recessive alleles. The only way I'm
going to get 500, is in this heterozygous condition.
'because I don't have any homozygous recessive. So
that means, to get 500 recessive alleles, I've gotta
have 500 heterozygous individuals so that I get
500 of these. 500 heterozygous individuals then makes up
500 of my dominants, as well. I need
1,500 total. So subtract 500 from 1,500. I need
1,000 more alleles. 500 in this box takes care
of 1,000, right? Because I get 500 of both.
500 of the first dominant and 500 of the second
dominant. Easy, huh? Or hard, depending on your perspective. I
really hope you understand this. If you don't, if you
have trouble, it's okay. There's not a lot of this course
that's going to deal with too much allelic frequencies. But I
do want to focus on it a bit more for the
rest of the lesson. If you can really get a
grasp of these allelic frequencies, you're going to have a whole new
appreciation for understanding gene flow, genetic
drift, and looking at huge populations of
individuals. And seeing how alleles travel
through. And can be shaped by evolution.