Hardy-Weinberg Equilibrium
and Population Genetics
Population Genetics
• Individuals do NOT
evolve; populations
do
▫ A population is a
group of
interbreeding
organisms in a
geographic area
Population Genetics
• A population evolves when individuals with
different genotypes survive or reproduce at
different rates
▫ This changes the gene pool’s allele frequency
Allele Frequency
• Proportion of the gene pool composed of a
specific allele
= # of copies of the allele in the population
sum of all alleles in the population
• Range from 0 to 1.0
Microevolution
• A generation to generation change in a
population’s allele frequencies
Conditions of Hardy-Weinberg
• Allele frequencies will not change between
generations
▫ Evolution WILL NOT occur
• All five conditions must be met!
• These are ideal conditions and do not
occur in nature
Hardy-Weinberg #1
• 1. Mating must be random
▫ Individuals do not preferentially choose mates
with certain phenotypes
Hardy-Weinberg #2
• Population size
must be LARGE
▫ In small
populations, allele
frequency is more
likely to change due
to genetic drift
Hardy-Weinberg #3
• No gene flow
▫ No migration which could potentially add or
remove alleles from the gene pool
Hardy-Weinberg #4
• No Mutations
▫ No changes to alleles
and no new alleles
added
Hardy-Weinberg #5
• No natural
selection
▫ No alleles will
be favored
Hardy-Weinberg Equilibrium
• If all 5 conditions hold true, allele frequencies in
the population do not change
• In a population with only 2 alleles at a locus:
▫ p = frequency of the dominant allele (A)
▫ q = frequency of the recessive allele (a)
• Therefore p + q = 1.0
Hardy-Weinberg Equilibrium
• If all 5 conditions hold true, allele frequencies in
the population do not change
• Allele frequency
▫p + q = 1
• Genotype frequency
▫p2 + 2pq + q2= 1
Solving Hardy-Weinberg Problems
• Always start by calculating q2
• Calculate the proportion of organisms with the
recessive phenotype. (Black is recessive)
Solving Hardy-Weinberg Problems
• Calculate q
• Take the square root of q2
Solving Hardy-Weinberg Problems
• Calculate p
Solving Hardy-Weinberg Problems
• Calculate 2pq
• 2pq gives you the proportion of heterozygotes
Solving Hardy-Weinberg Problems
• Calculate p2
• 2 ways to do this…
Check your work!
• If p2 = 0.25, 2pq=0.5 and q2 = 0.25,
then p2 + 2pq + q2 = 1
• 25% AA
• 50 % Aa
• 25 % aa
Hardy-Weinberg Problems
• In a certain population of 1000 fruit flies, 640
have red eyes while the remainder have sepia
eyes. The sepia eye trait is recessive to red eyes.
How many individuals would you expect to be
homozygous for red eye color?
• Hint: start with q2!
Hardy-Weinberg Problem
• 16% of a frog population has smooth skin. This
trait is recessive to spotted skin. What
percentage of frogs in this population are
heterozygous and homozygous dominant for
spotted skin?