Introduction

To keep things relatively simple, we'll spend much of our time in this course taling about variation at a single genetic locus, even though alleles at many different loci are involved in expression of most morphological or physiological traits. We'll spend about three weeks in mid-October studying the genetics of quantitative variation, but until then you can asssume that I'm talking about variation at a single locus unless I specifically say otherwise.

When I talk about the genetic composition of a population, I'm referring to three aspects of variation within that population:¹

1. The number of alleles at a locus.

2. The frequency of alleles at the locus.

3. The frequency of genotypes at the locus.

It may not be immediately obvious why we need both (2) and (3) to describe the genetic composition of a population, so let me illustrate with two hypothetical populations:

	$A_1A_1$	$A_1A_2$	$A_2A_2$
Population 1	50	0	50
Population 2	25	50	25

It's easy to see that the frequency of $A_1$ is 0.5 in both populations,² but the genotype frequencies are very different. In point of fact, we don't need both genotype and allele frequencies. We can always calculate allele frequencies from genotype frequencies, but we can't do the reverse unless $\dots$