Population genetics

Population genetics

Study of genetic composition of a group of individuals

• Frequency of different alleles in population

• How genetic composition changes in a population over time

Population

Group of interbreeding, sexually reproducing individuals that share a common gene pool

Gene Pool

Total of all alleles carried in all members of a population

• can be described by calculating genotypic AND allelic freq

Genotype Frequency

Proportion of a particular genotype in a population

• # genotype individuals / total individuals

Calculate Allelic Frquency

Given: # of individuals w/ GG, Gg, and gg genotypes

Proportion of a particular genotype in a population

1. Calculate from genotypic frequencies

p = f(GG)+½ (Gg)

q = f(gg) +½ (Gg)

2. Calculate from individiual #

p = (GG ind. *2) + (Het ind.)/ (Total ind.*2)

Hardy-Weinberg Equilibrium + 5 assumptions + 2 Conclusions

If pop. is in HWE, allele freq are not changing over time

1. very large population

2. random mating

3. No new mutations

4. no migration

5. no natural Selection

1. Allele frequencies are not changing one gen to next

2. Genotype frequencies can calculated from allele frequencies

Calculate genotypic frequencies given allelic frequencies (MUST BE IN HWE)

• p² = homo dom

• 2pq = het

• q² = homo rec

How to determine HWE?

1. Calculate allelic frequencies (p,q)

2. Calculate expected HWE genotypic frequencies (p², 2pq, q² )

3. Calculate chi squared value = sum (obs-exp)²/exp → Chi squared value

4. Degrees of freedom = # of genotypes - # of alleles

5. Go to chart to find p value

   1. p<0.05 → population is not in HWE

   2. p<0.05 → population is in HWE

HWE genotypic freq formulas: three alleles

(P+ q+ r)²

p^₂ + 2pq + q^₂ + 2pr + p^₂ + 2qr

HWE genotypic freq formulas: x linked alleles

Females: p², 2pq, q²

• Genotype freq is normal

Males: p, q

• Genotype freq is =allele freq bc they only have one allele of X linked gene

Nonrandom mating

• positive assortative mating (like mates like)

• negative assortative mating (unlike individuals mate)

• inbreeding

How do positive/negative assortative mating vs. inbreeding affect allelic frequencies?

Positive/negative assortative → affect genes associated w/ a particular trait

Inbreeding → affects all genes

Inbreeding coefficient (F)

Probability that 2 alleles are identical by descent
F = 0 → mating random

F = 1 → all alleles are identical by descent

How does Inbreeding impact genotypic frequencies?

1. reduces proportion of heterozygotes

2. increases proportion of homozygotes according to these eq

1. f(AA) = p² + Fpq

2. f(Aa) = 2pq - 2Fpq

3. f(aa) = q² + Fpq

Eventually all genotypes will be homozygous

If neither homozygote has an advantage → both alleles will remain in pop

Inbreeding depression

increased appearance of recessive lethal and deleterious traits in population

Gene Mutation

necessary for genetic variation that leads to evolution

Migration

• Causes gene flow between population

• Causes gene pools of diff populations to become more similar

• Adds genetic variation to populations

If population I → population II, pop II becomes more _____ & ______

like pop I ; genetically diverse

Genetic Drift

changes to alle freq. bc there is chance involved in gamete selection

• Happening always in all populations

• Greatest effect on small pop

Sampling Error that leads to genetic drift can arise because of… ()

Resource limitations

founder effect

Genetic bottleneck

How do resource limitations cause genetic drift?

Not enough space, food, etc leads to to reduced population size

How does Founder Effect cause genetic drift

1. Isolated population established by small # of individuals

2. Allele frequencies change bc founders carry only a fraction of sales from original population (limited gene pool)

How does Genetic Bottleneck cause genetic drift

1. Population undergoes drastic size reduction

2. May be due to hunting or disease

Effects of genetic drift

1. random changes allele frequency w/in population

2. Reduced genetic variation W/in a population

3. Genetic divergence between populations

Natural Selection

Differential reproduction of certain genotypes in a population

• Driven by interactions between phenotypes and environment

• Eliminates individuals w/ lower fitness

• Individuals w/ higher fitness are more likely to parent next gen

Fitness

relative reproductive success of a genotype

Values between 0 and 1

• High value → genetic variations help individual survive and reproduce

calculating relative fitness

1. Calculate avg # of progeny = Surviving Progeny/ genotype ind. #

2. Relative fitness avg # progeny / biggest avg progeny #

How do Genotype Frequencies in Next gen change by relative fitness factor?

1. Multiple genotype frequencies (p²,q², 2pq) by relative fitness (from avg progeny)

2. Sum values → mean fitness

3. use mean fitness to normalize values e.g. (p² * W_AA)/ ŵ → new expected genotype frequences

   1. can be used to calculate allelic frequencies