Hardy-Weinberg Equilibrium
Honors Biology
Is evolution occurring right now?

How might a scientist
tell if evolution is
occurring within a
population?
Hardy-Weinberg Theorem
Biologists use models
to study populations
 Hardy-Weinberg
equilibrium is a type of
model


Provides a framework for
understanding how
populations evolve
The Hardy-Weinberg Theorem

Uses frequencies of alleles in a
population’s gene pool to
determine if (micro)evolution
is occurring


If allele frequencies change over
time, the population is evolving
If allele frequencies are stable,
the population is at equilibrium
Hardy-Weinberg Equilibrium Conditions
Genotype frequencies stay the same (no
microevolution) if all 5 conditions are met:
1. Very large population size
2. Isolation from other populations
3. No Mutations
4. Random Mating
5. No natural selection

Hardy-Weinberg Equation

p+q=1



p = frequency of dominant allele
q = frequency of recessive allele
p2 + 2pq + q2 = 1



p2 = frequency of homozygous dominant genotype
(AA)
2pq = frequency of heterozygous genotype (Aa)
q2 = frequency of homozygous recessive genotype
(aa)
Solve this story problem
In pigs, the allele for black skin is
recessive to pink skin.
 B = pink; b = black
 Calculate q2 for the illustration shown
below.
 q2 = 0.25 (4/16)


Find q.


Find p.


q = 0.5
p = 0.5
Find 2pq.


2pq = 0.5
This means 50% of the pink pigs are
heterozygous.
Solve this story problem
In a certain flock of sheep, 4 percent of
the population has black wool and 96
percent has white wool.
 If black wool is a recessive trait:




What percentage of the population is
homozygous black? (q2)
What percentage of the population is
heterozygous for this trait? (2pq)
What percentage of the population is
homozygous white? (p2)