Species exist as one or more populations. The concepts of gene pool and allele frequency. The Hardy-Weinberg principle. The conditions under which the principle applies. Candidates should be able to calculate allele, genotype and phenotype frequencies from appropriate data and from the Hardy-Weinberg equation, p2 + 2pq + q2 = 1 where p is the frequency of the dominant allele and q is the frequency of the recessive allele. Candidates should understand that the Hardy-Weinberg principle provides a mathematical model which predicts that allele frequencies will not change from generation to generation.

A gene pool is all the genes that exist in a population. So say we take a population of rabbits and they are all albino then we know that the genes in this pool are albino. Say we take a population of rabbits and there are some of each colour then we know the gene pool has grey, chinchilla, himalayan and albino in it.

In this second population of rabbits there will be higher numbers of some colours than others. The amount of times an allele crops up is known as allele frequency.

The Hardy-Weinberg principle is an equation that allows us to work out the frequency of alleles in a population.

It states that the frequency of alleles will stay constant from one generation to the next provided that: there is genetic isolation; the population is large; mating is random; there are no mutations; there's no natural selection; there's no migration.

Here is how it works:

We use the short hand p to represent the dominant allele and q to represent the recessive allele.

We know that if you added up all of the dominant alleles and all of the recessive alleles the total would be all the alleles (100%) so:

p+q= 1.0

We also know that if you took all the possible genotype and added them up you would get all the alleles:
So DD + Dd + dD + dd= 100% or

p2 + 2pq + q2= 1.0

From these two equations, if given the information on one phenotype, you are able to work out different things about a population. Here is  a worked example:

A sample of a population shows that 36% of a population is homozygous recessive (aa). Find the frequency of the dominant phenotype.

So the first thing you want to do is work out what they are asking you for, and in this case it would be any genotype with the dominant gene in so Aa and aa, so 2pq plus p2. And what have they given you: aa which is  q2. I like to then write out a list of what I have and what I need:

q=
p=
p2=
2pq=
q2= 0.36
p2 + 2pq=

As stated earlier we know that p+q= 1.0 (or 100%) so we know that 1.0-q will give us p:

Square root of q2= sqrt 0.36 = 0.6
p= 1.0-0.6= 0.4

So we now have:

q= 0.6
p= 0.4
p2=
2pq=
q2= 0.36
p2 + 2pq=

So to work out p2 we square p= 0.16
and to work out 2pq we do 2 x p x q = 0.48

and to work out the answer we do 0.16 + 0.48= 0.64