All of the cases that we have examined have had a single gene with two alleles. But what about genes that have three or more alleles? This is dealt with in the same way, but the determination of allele frequencies becomes a bit more involved.
You previously learned about the human ABO blood group system. This was used as an example of a gene with multiple alleles (IA , IB , i ) and codominance because both alleles are expressed in AB heterozygous individuals. The O allele (designated as i ) is recessive to the IA and IB alleles. Now we’ll work a problem for a system with three alleles. In a human population, the IA allele has a frequency of 0.3 and the IB allele has a frequency of 0.1. Pull out a piece of paper and a pencil and write down these numbers.
First, calculate the frequency of the O allele. Remember, the allele frequencies for a particular gene must add up to one. Now that you have these three allele frequencies, determine the frequencies for all of the possible genotypes in the population. When you are finished, these should also add up to one. If it is helpful, the H-W equation that you would use for three alleles is p+q+r=1, and the equation for the genotypes is p 2 + 2pq + q 2 +2pr +2qr + r 2 = 1.
Population Genetics Part 1 VoiceThread Transcript
You should now appreciate how the Hardy-Weinberg equation can be used to study the genetic composition of a population. But how does this fit into evolution studies?
The answer is that changes in allele frequencies are the most fundamental indication that evolution is occurring in a population. If two populations of the same species have the same allele frequency for a given gene, it can be concluded that evolutionary forces are not operating on that gene (at least at that time). On the other hand, if the allele frequencies are different for a given gene, then evolutionary forces may be operating on that population.
If allele frequencies remain stable between generations, then the population is considered to be in Hardy-Weinberg equilibrium.