Evolution__3rd_Edition

96 PART 2 / Evolutionary Genetics
We define genotype frequency ...
. . . and gene frequency
5.1 Population genetics is concerned with genotype
and gene frequencies
The human genome, on current estimates, contains something like 30,000 gene loci.
Let us focus on just one of them a on a locus at which there is more than one allele,
because no evolutionary change can happen at a locus for which every individual in the
population has two copies of the same allele. We shall be concerned in this chapter with
models of evolution at a single genetic locus; these are the simplest models in population
genetics. Chapters 8 and 9 discuss more complex models in which evolutionary
change occurs simultaneously at more than one locus.
The theory of population genetics at one locus is mainly concerned to understand
two closely connected variables: gene frequency and genotype frequency. They are easy to
measure. The simplest case is one genetic locus with two alleles (A and a) and three
genotypes (AA, Aa, and aa). Each individual has a genotype made up of two genes at the
locus and a population can be symbolized like this:
Aa AA aa aa AA Aa AA Aa
This is an imaginary population with only eight individuals. To find the genotype
frequencies we simply count the numbers of individual with each genotype. Thus:
Frequency of AA = 3/8 = 0.375
Frequency of Aa = 3/8 = 0.375
Frequency of aa = 2/8 = 0.25
In general we can symbolize genotype frequencies algebraically, as follows.
Genotype AA Aa aa
Frequency P Q R
P, Q, and R are expressed as percentages or proportions, so in our population,
P = 0.375, Q = 0.375, and R = 0.25 (they have to add up to 1, or to 100%). They are
measured simply by observing and counting the numbers of each type of organism in
the population, and dividing by the total number of organisms in the population (the
population size).
The gene frequency is likewise measured by counting the frequencies of each gene in
the population. Each genotype contains two genes, and there are a total of 16 genes per
locus in a population of eight individuals. In the population above,
Frequency of A = 9/16 = 0.5625
Frequency of a = 7/16 = 0.4375
Algebraically, we can define p as the frequency of A, and q as the frequency of a. p and q
are usually called “gene” frequencies, but in a strict sense they are allele frequencies: they
are the frequencies of the different alleles at one genetic locus. The gene frequencies can
be calculated from the genotype frequencies:
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