Population Genetics and the Hardy-Weinberg Principle
Most genetics research focuses on the structure of genes on chromosomes, the function of genes, and the process
of genetic transmission from parent to offspring.
Population genetics instead focuses on the overall gene pool in a population of interbreeding organisms - that is,
the frequency of all alleles of all genes in the population - and whether the gene pool may be changing across
generations in a population. Population genetics examines the relationship among genotype frequencies in a
population, allele frequencies in its gene pool, and factors that can change these frequencies over time.
Genetic Equilibrium and the Hardy-Weinberg Principle
A population is in genetic equilibrium when allele frequencies in the gene pool remain constant across generations.
A gene pool will be in equilibrium under the following conditions:
ā¢ the population is very large
ā¢ individuals in the population mate randomly
ā¢ there is no migration into or out of the population
ā¢ natural selection does not act on any specific genotypes
ā¢ males and females have the same allele frequencies [vs. individuals are diploid and reproduce sexually]
ā¢ no mutations occur
In 1908 Godfrey Hardy and Wilhelm Weinberg, working independently, specified the relationship between
genotype frequencies and allele frequencies that must occur in such an idealized population in equilibrium. This
relationship, known as the Hardy-Weinberg principle, is important because we can use it to determine if a
population is in equilibrium for a particular gene.
Population Genotypes and Alleles
The Hardy-Weinberg principle applies to individual genes with two alleles, a dominant allele and a recessive allele.
A population with such a gene can be described in terms of its genotype numbers - the number of individuals with
each of the three resulting genotypes - or in terms of the three genotype frequencies. The frequency of each
genotype is the number of individuals in the population with that genotype divided by the total number of
individuals in the population, as displayed in Table 1. (Note that total genotype frequencies sum to 1.0.)
Table 1. Genotype numbers and genotype frequencies in a hypothetical p opulation.
Genotypes
Number with these Genotypes
Genotype Frequencies
AA
180
0.36
Aa
240
0.48
aa
80
0.16
Total
500
1.0
Calculating Allele Frequencies
We can also describe a population, somewhat more abstractly, in terms of its allele frequencies. The frequency of
an allele is defined as the total number of copies of that allele in the population divided by the total number of
copies of all alleles of the gene. We can calculate population allele frequencies from genotype numbers.
The total number of dominant A alleles in our population equals 600, which is the sum of:
- the number of AA individuals times 2 (the number of A alleles per individual) = 180 x 2 = 360
- the number of Aa individuals (times 1, the number of A alleles per individual) + 240
600
The total number of all alleles of the gene equals 1000, which is 2 times the number of individuals in the
population (since the individuals are diploid).
