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

Population: 795 AA 190 Aa 15 aa

Genotype frequencies:

AA Aa aa

795/1000 190/1000 15/1000

0.795 AA 0.190 Aa 0.015 aa

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

These terms are based on mating that occurs when gametes combine at random

pA / qa
pA / p2AA / pqAa
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 / Genotypes / Observed / Expected / d / d2 / d2/exp
Normal / AA / 795 / 792.1 / 2.9 / 8.41 / 0.0106
Some resistance to HIV / Aa / 190 / 195.8 / -5.8 / 33.64 / 0.172
Resistant to HIV / 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.

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