HARDY WEINBERG EQUILIBRIUM
 Deals w/ distribution of genes within a population – says that frequency of an allele in a gene pool will
remain constant unless acted upon by other agents
 In nature, there are two opposing forces in operation – forces that produce a stable, unchanging population,
and forces that cause a population to change (leading perhaps to evolution).
 Hardy Weinberg applies to only stable populations, and there are 5 conditions that must be met in order
for H-W equilibrium to be maintained:
1.
2.
3.
4.
5.
Large population size
No migration in or out
No mutations
Random mating
No natural selection
 Now, in nature these conditions are rarely met, so why use H-W? You can use H-W frequencies as a basis
for comparison when studying populations in which gene pools are changing. If a population’s frequencies
are changing, then you can try to determine which of the above conditions is not being met.
The Equations:
Equation for allele frequency:
p+q =1
p = frequency of ___________________ allele
q = frequency of ___________________ allele
Equation for genotype frequency:
p2 + 2pq + q2 = 1
p2 = frequency of _________________________ genotype
2pq = frequency of _________________________ genotype
q2= frequency of __________________________ genotype
Practice Problems:
1. In field mice, gray coat color (G) is dominant and black (g) is recessive. Within a given population, the
frequency of the dominant allele is 80% (or .80). Find the distribution of all three genotypes in this
population.
2. In a population of 100 field mice, 25 are black. What are the frequencies of the dominant and recessive
alleles? What are the three genotypic frequencies for this population?
3. If the frequency of a dominant allele is present at 60%, what will be the expected percent of the
population with the hybrid genotype?
4. In a population of field mice, the frequency of the recessive allele of black coat color is present at 30%.
What is the distribution of the homozygous recessive and heterozygous genotypes?
5. In a population of 400 rabbits, 300 have the dominant allele (W) for white fur.
a. What is the frequency of the recessive allele (w) for black fur in this population?
b. In the same population, what percentage of rabbits would be hybrid for white fur?
6. The percentage of people with free earlobes (dominant) is 84% while 16% of the people have the
recessive allele. Find the distribution of all genotypes for this population.
7. In Drosophila the allele for normal-length wings is dominant over the allele for vestigial wings (tiny
stubs that can’t be used for flight.) In a population of 1,000 flies, 360 show the recessive phenotype.
How many individuals would you expect to be homozygous dominant? How many would you expect to
be heterozygous?
8. The allele for free earlobes is dominant over the allele for attached earlobes. In a population of 500
individuals, 25% show the recessive phenotype. How many individuals would you expect to be
homozygous dominant for this trait? Heterozygous?
9. The allele for the hair pattern called widow’s peak is dominant over the allele for no widow’s peak. In a
population of 1,000 individuals, 510 show the dominant phenotype. How many individuals would you
expect of each of the possible three genotypes for this trait?