Non-random Mating: Inbreeding and Population Structure

What is non-random mating?

Mating in which individuals do not choose mates randomly.

What Hardy-Weinberg assumption is violated by non-random mating?

The assumption that mating occurs randomly.

What is assortative mating?

Mating between individuals with similar phenotypes.

What is disassortative mating?

Mating between individuals with dissimilar phenotypes.

What is inbreeding?

Mating between individuals related by ancestry.

What is inbreeding avoidance?

Preferentially avoiding mating with relatives.

What is population structure?

The existence of genetically differentiated subpopulations within a larger population.

What is sexual selection?

Choosing mates based on particular traits.

What does identical by descent mean?

Two alleles originated from the same ancestral allele.

Does inbreeding directly change allele frequencies?

No. Inbreeding alone changes genotype frequencies but not allele frequencies.

How does inbreeding affect homozygosity?

It increases homozygosity.

How does inbreeding affect heterozygosity?

It decreases heterozygosity.

What is inbreeding depression?

Reduced fitness associated with increased expression of deleterious recessive alleles.

Why can inbreeding increase expression of harmful recessive alleles?

It increases the probability that individuals inherit two copies of the same recessive allele.

What is the inbreeding coefficient F?

A measure of the reduction in observed heterozygosity compared with expected heterozygosity under random mating.

What is the formula for F?

F = 1 - (Ho / He).

What does Ho represent?

Observed heterozygosity, or the observed fraction of heterozygous individuals.

What does He represent?

Expected heterozygosity under Hardy-Weinberg equilibrium.

What is the formula for expected heterozygosity?

He = 2pq.

What does F = 0 indicate?

Observed heterozygosity equals expected heterozygosity; no heterozygote deficiency.

What does F = 1 indicate?

Complete heterozygote deficiency, corresponding to full inbreeding in the lecture model.

How do you calculate Ho using F?

Ho = He(1 - F).

What is the observed heterozygosity formula using p, q, and F?

Ho = 2pq(1 - F).

What is the genotype frequency of AA with inbreeding?

GAA = p² + pqF.

What is the genotype frequency of Aa with inbreeding?

GAa = 2pq - 2pqF.

What is the genotype frequency of aa with inbreeding?

Gaa = q² + pqF.

How much heterozygosity is lost due to inbreeding?

2pqF.

How much does each homozygote frequency increase due to inbreeding?

pqF.

Why does inbreeding have a large effect on rare recessive homozygotes?

Their Hardy-Weinberg frequency q² is extremely small, so adding pqF can cause a large proportional increase.

If q = 0.001, what is the Hardy-Weinberg frequency of aa?

q² = 0.000001, or approximately one in a million.

If q = 0.001 and F = 0.01, what is the frequency of aa?

Gaa = q² + pqF ≈ 0.000011.

How much does F = 0.01 increase rare recessive homozygotes in the lecture example?

Approximately 11-fold compared with Hardy-Weinberg expectations.

Who is the historical example of inbreeding depression?

King Charles II of Spain from the Habsburg dynasty.

What was the approximate inbreeding coefficient of Charles II?

F = 0.25.

What does the Charles II example illustrate?

Repeated mating among relatives can greatly increase the probability of expressing deleterious recessive alleles.

What causes genetic differences among subpopulations?

Genetic drift, sometimes natural selection, and limited gene flow.

What does gene flow do to differences between subpopulations?

It generally reduces differences by making allele frequencies more similar.

Why can mating within local subpopulations be considered a form of inbreeding?

Individuals within a subpopulation tend to be more genetically similar than individuals sampled from the total population.

In the extreme BB and bb subpopulation example, what is observed heterozygosity?

Ho = 0 because all individuals are homozygous.

In the extreme BB and bb example with equal subpopulation sizes, what is expected heterozygosity for the total population?

He = 2(0.5)(0.5) = 0.5.

What are F-statistics?

Fixation indices that measure reductions in heterozygosity at different population levels.

Who developed F-statistics?

Sewall Wright.

What do I, S, and T stand for in F-statistics?

I = individual; S = subpopulation; T = total population.

What does FIS measure?

Reduction in observed individual heterozygosity relative to expected heterozygosity within subpopulations.

What does FIT measure?

Reduction in observed individual heterozygosity relative to expected heterozygosity in the total population.

What does FST measure?

Reduction in expected heterozygosity due to subpopulation structure; genetic differentiation among subpopulations.

What is the formula for FST using heterozygosity?

FST = 1 - (HexpS / HexpT).

What is HexpS?

Expected heterozygosity averaged across subpopulations.

What is HexpT?

Expected heterozygosity calculated for the total population.

What does FST = 0 indicate?

No genetic differentiation among subpopulations at the locus.

What are Wright's FST interpretation guidelines for biallelic loci?

0–0.05 little or none; 0.05–0.15 moderate; 0.15–0.25 great; above 0.25 very great differentiation.

What relationship did the human population graph show?

Greater geographic distance along historical migration routes generally corresponds to greater pairwise FST.

What is the island-model equation for FST?

FST = 1 / (4Nm + 1).

What does Nm represent?

Number of successful migrants per generation in the island model.

What happens to FST when migration increases?

FST decreases because gene flow reduces genetic differentiation.