Biology 212 General Genetics
Spring 2007
Lecture 26: Population Genetics I
Reading: Chapter 14 pp. 500-506
Lecture outline:
1. Definitions
2. Genotype frequencies and allele frequencies
3. Hardy-Weinberg Equilibrium
Lecture
1. Definitions
Population genetics: applying genetic principles to groups of individuals from the same
species.
Population: group of individuals of some species living within a prescribed
geographical area.
Gene pool: complete set of genetic information contained within the individuals in a
population.
Genotype frequencies: how often a particular genotype occurs in a population;
expressed as a fraction.
Allele frequencies: the frequency of a particular allele among all versions of a particular
gene.
2. Genotype frequencies and allele frequencies
Example: Analysis of a particular population in France for alleles of the CCR5 receptor
gene.
CCR5 receptor: Chemokine receptor; protein on the surface of lymphocytes that binds
chemokines, a signaling molecule. Also serves as the coreceptor for the HIV virus.
Human populations are polymorphic for the CCR5 receptor
A=normal allele; susceptible to HIV infection
a=Δ32 deletion
 removes 32 bp from gene
 creates a frameshift mutation in region encoding receptor protein
 individuals with mutant receptor are less susceptible to HIV infection
1000 French people genotyped for CCR5
1
Population:
795 AA
190 Aa
15 aa
AA
795/1000
Aa
190/1000
aa
15/1000
0.795 AA
0.190 Aa
0.015 aa
Genotype frequencies:
Allele frequency = frequency homozygotes + 1/2 frequency heterozygotes
Allele frequency of A = frequency AA + 1/2 frequency Aa
0.795 + 1/2 (0.190) = 0.89
Allele frequency of a = frequency aa + 1/2 frequency Aa
0.015 aa + 1/2 (0.190) = 0.11
sum of allele frequencies = 1
frequency (A) + frequency (a) = 1
0.89 + 0.11 =1
3. Hardy Weinberg equilibrium


derived by G. Hardy and W. Weinberg independently in 1908
a mathematical prediction of genotype frequencies and allele frequencies in
populations based on
o Mendel's laws
o Random mating=organisms in population form mating pairs independent
of genotype
o No natural selection
o No mutation
o No migration
o No genetic drift: random fluctuations in allele frequencies due to chance
Let p = allele frequency for A
Let q = allele frequency for a
Let the genotypes equate to the following terms
AA
Aa
aa
p2
2pq
q2
2
These terms are based on mating that occurs when gametes combine at random
pA
p2AA
pqAa
pA
qa
qa
pqAa
q2aa
Predictions of the H-W equilibrium
 If assumptions are met, H-W equilibrium will be established in one
generation
 Once a population is in H-W equilibrium allele frequencies remain constant from
one generation to the next.
Use of a chi-square test to determine whether population has reached H-W
equilibrium
 French population with CCR5 receptor polymorphism
Phenotypes
Normal
Some
resistance
to HIV
Resistant
to HIV
Genotypes Observed
AA
795
Aa
190
Expected
792.1
195.8
d
2.9
-5.8
d2
8.41
33.64
d2/exp
0.0106
0.172
aa
15
12.1
2.9
8.41
0.695
1000
1000
χ2=0.878

Note: degrees of freedom is defined differently here. Not number of phenotypes 1. Since p and q are only variables once p is known, q = 1 - p, therefore only one
degree of freedom.

Chi-square test will identify large deviations in expected genotype frequencies
compared to that expected by H-W equilibrium.
3