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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 nonpersistent, 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.