Alleles of a gene don't all exist at an equal frequency within a population. Some alleles are far more common than others, and these frequencies may change over time in response to external forces. Population genetics studies the frequencies of alleles within a population, and the probabilities of inheriting a particular trait based on those frequencies. This branch of genetics also looks at the forces that can affect allele frequency.
Up to this point in the course, we have used the principles of genetics to calculate the probabilities of an individual inheriting a particular trait from its parents. What we didn't consider in such calculations is how frequently such a trait might occur in an entire population. To illustrate: two pigmented parents, if both heterozygous for albinism, have a 1/4 chance of having an albino child. But how often does albinism occur in a whole population? Population Genetics can be used to answer such questions.
Before going any further, make sure you know and understand the definitions of population and gene pool as they apply to population genetics.
Population genetics measures the frequency of alleles of a particular gene (in other words, how many copies of that allele are present in the population?). How is allele frequency determined? It's not exact, but there are several ways of doing so depending on what type of population you are studying. For a population of humans, pedigree analysis and molecular analysis (if the molecular nature of the allelic forms is known) can be done. For populations of fruit flies, testcrosses can be used to determine allele frequency.
As an example, let's consider a gene with only two alleles, A and B. In a population of 100 individuals, there will be 200 copies of this gene (two copies each, right?). If allele A is present in 80 copies, then the frequency of A is:
80/200 = 0.4 (40%)
Allele B will therefore be present in 120 copies in the population, so the frequency of allele B is:
120/200 = 0.6 (60%)
Note that the two allele frequencies add up to 1:
0.4 + 0.6 =1.0
This is a law of population genetics: The sum of all allele frequencies will always be equal to 1. This is because 1 represents the frequency of all possible alleles within the population. If we define the letter p as the frequency of one allele and the letter q as the frequency of the other allele, then this can be expressed as
p + q = 1
If there are three alleles for a particular gene, then the frequencies of all three will add together to equal 1, etc. (We can define extra letters to cover the extra alleles as need be.)
How useful are allele frequencies? For starters, we can use allele frequencies to determine how frequently each different genotype will occur in the next generation of a population. To do this, we just plug the data into a Punnet square:
The numbers in parentheses indicate the frequencies of each allele or combination of alleles. For each allele combination, the frequency is the product of the frequencies of the two alleles (according to the product law). The frequency of homozygous A is 0.16 or 16%, the frequency of homozygous B is 0.36, or 36%, and the frequency of heterozygotes is 0.48 (2 x 0.24), or 48%. Note that once again, these frequencies add up to 1, as they must. Using the p and q nomenclature outlined above, we can state this as a mathematical formula:
p2 + 2pq + q2 = 1
This formula will come in extremely useful, as we shall see.