Population Genetics

Refresher…

• What is a population?

• What is a species?

• What is an allele?

• What is a gene pool?

Genetic Diversity

• Genetic diversity is generally high within

populations

– High degree of heterozygosity

– Genetic diversity may not be easily detectable

• Two methods to detect genetic diversity:

– Artificial selection

– Nucleotide sequencing

Adh gene in Drosophila

14 18

11

Of the 14 exon mutations, 13 are silent mutations

Only one changes an amino acid

Human CFTR gene

• >1900 different mutations identified in this

gene

– Missense

– Amino acid deletions

– Nonsense mutations

– Frameshifts

– Splice defects

• Cystic fibrosis transmembrane

conductance regulator

– Most common genetic disease in Caucasians

Clicker Question: True or False:

High amounts of genetic diversity

are always accompanied by high

phenotypic variation in a population

• A) True

• B) False

Microevolution

• Changes in allele frequencies in a

population that do NOT lead to speciation.

– No reproductive isolation

• Key elements of population genetics:

– Allele frequencies in a population

– Genotype frequencies in a population

– How these frequencies change from one

generation to the next

Hardy-Weinberg Law • Examines allele and genotype frequencies

in an “ideal” population:

– Infinitely large

– Randomly mating

– Not subject to evolutionary forces

• No migration

• No mutation

• No selection (artificial or natural)

Hardy-Weinberg

• Refresher of Mendelian inheritance and

the multiplication rule

• Consider an autosomal gene with two

alleles: A and a

• A has a frequency of 0.7 (=70%)

• a has a frequency of 0.3 (=30%)

• A + a = 1.0 (=100%) – there are no other

alleles to consider

Hardy-Weinberg

Figure 22-3

0.49 + 0.21 + 0.21 + 0.09 = 1.00

Hardy-Weinberg

• What is the frequency of

alleles in the offspring

generation?

• AA: 0.49

• aa: 0.09

• Aa: 0.42

• A = 0.49 + (½)*(0.42) = ?

• a = 0.09 + (½)*(0.42) = ?

Figure 22-3

Hardy-Weinberg: Generic

• First allele is p; second allele is q

• Homozygous: pp = p2 or qq = q2

• Heterozygous: pq

• In typical matings, heterozygotes occur

twice as often as either homozygote type

• So… p2 + 2pq + q2 = 1.0

Hardy-Weinberg: Predictions

• Allele frequencies do not change from

generation to generation

– No speciation!

• Genotype frequencies in offspring can be

predicted from allele frequencies

Hardy-Weinberg: Applications

• When real-life allele frequencies do

change, scientists can determine what

factors may be responsible

• Neutral genes can be identified in

populations

Hardy-Weinberg: Implications

• Dominant traits do not increase from

generation to generation

• Genetic variability remains in populations

once established

• Knowing frequency of one genotype

allows calculation of frequency of other

genotypes.

– Can calculate frequency of disease carriers

when we only know the frequency of affected

individuals

In a group of 125 students, 88 can taste

PTC; 37 cannot (homozygous recessive).

Calculate frequency of T and t alleles in this

population, and frequency of the genotypes.

Hardy-Weinberg in Human

Population • Example: CCR5 gene product allows HIV-

1 to enter cells (allows infection)

• CCR5-1 is normal allele

• CCR5-D32 is “mutant” allele, resistant to

HIV infection

• Heterozygotes can be infected, but

disease progresses more slowly than

homozygous “normal” patients

CCR5 gene data

Hardy-Weinberg Equilibrium • Do the offspring allele and genotype

frequencies match those of the parental

generation?

– Generate prediction for offspring based on

parents (calculate “expected”)

– Measure offspring frequencies (“observed”)

– Use c2 analysis to determine whether observed

frequencies match expected frequencies

• Population is in H-W equilibrium if there are

no changes in frequencies from generation

to generation

Hardy-Weinberg Equilibrium • Reasons a population is NOT in H-W

equilibrium:

– Selection

– Genetic drift

– Mutation

– Migration

– Nonrandom mating