Hardy-Weinberg: Nonrandom mating and Inbreeding

• The Hardy-Weinberg principle assumes random mating, but this is often violated in reality.
• Nonrandom mating falls into two categories: assortative and disassortative.
• Assortative Mating: Individuals prefer mates with similar genotypes/traits.
• Disassortative Mating: Individuals prefer mates with different genotypes/traits.

Understanding Inbreeding

• Inbreeding: Individuals mate with close genetic relatives rather than choosing mates randomly.
• In inbred populations, genes are not combined at random.
• Offspring inherit gene copies that may be Identical by Descent (IBD)

Identity by Descent (IBD)

• IBD: Two copies of a gene in an individual came from the same ancestor.
• IBD is different from two genes simply having the same sequence

Self-Fertilization (Selfing)

• Self-fertilization (selfing): An individual fertilizes its own gametes.
• Selfing is common in flowering plants.
• Selfing affects genotype frequencies over generations, and is an extreme form of inbreeding.

Population Allele Example

• Population has two alleles, with A1 frequency (p) = 0.8 and A2 frequency (q) = 0.2.
• Hardy-Weinberg genotype frequencies: A1A1 = 0.64 (64%), A1A2 = 0.32 (32%), A2A2 = 0.04 (4%).

What Happens When the Population Starts Selfing?

• A1A1 individuals only produce A1A1 offspring
• A2A2 individuals produce A2A2 offspring
• A1A2 individuals produce offspring in a 1:2:1 ratio (25% A1A1, 50% A1A2, 25% A2A2).
• Each generation of selfing reduces heterozygotes (A1A2), increases homozygotes (A1A1 and A2A2).
• After generations of selfing heterozygotes become extremely rare.
• The population consists almost entirely of homozygotes.
• Allele frequencies (p and q) remain unchanged because no alleles are added/removed.

Key Takeaways

• Nonrandom mating breaks the Hardy-Weinberg assumption of random mate selection.
• Inbreeding: A form of assortative mating where individuals mate with relatives.
• Identity by descent: Two gene copies came from the same ancestor
• Selfing is the extreme form of inbreeding that leads to a rapid loss of heterozygotes, though allele frequencies remain stable.
• Inbreeding alone does not change allele frequencies, it only affects genotype distributions.

What is Inbreeding Depression?

• Inbreeding depression: Closely related individuals mate, leading to offspring with reduced fitness.
• Inbreeding depression is caused by increased homozygosity (offspring inherit two identical alleles from a common ancestor).

Genetic Mechanisms Behind Inbreeding Depression

• Overdominance (Heterozygote Advantage):
• Heterozygotes (two different alleles) have higher fitness than homozygotes.
• Inbreeding increases homozygosity, reducing heterozygotes, lowers overall fitness.
• Expression of Deleterious Recessive Alleles:
• Harmful recessive alleles exist at low frequencies in a population.
• In outbred populations, these alleles remain hidden in heterozygotes.
• In inbred populations, offspring are more likely to inherit two copies of a recessive allele, exposing harmful effects.

Why is Inbreeding Depression Important?

• Conservation Biology Concerns:
• Threatened populations are small and isolated, leading to higher inbreeding rates.
• Reduced genetic variation makes populations vulnerable to disease and environmental changes
• Habitat fragmentation worsens inbreeding by isolating groups, forcing mating between relatives.

Example: Inbreeding in Scandinavian Gray Wolves

• Wolves were extinct in Scandinavia by the 1960s.
• A few wolves migrated from Russia/Finland and formed a small population in 1983.
• Genetic diversity improved slightly with migration in 1991, leading to population growth to 300 by 2011.

Wright's F-Statistic (Inbreeding Coefficient)

• Wright's F-Statistic is used to measure the extent of inbreeding.
• High F-values mean more homozygosity and lower genetic diversity.
• The survival rate of wolf pups was negatively correlated with inbreeding (higher inbreeding = fewer surviving pups).
• Gene flow from Russian/Finnish wolves helped counteract inbreeding depression.

Wright's F-Statistic (Measuring Inbreeding)

• Wright's F-Statistic measures the probability that two alleles in an individual are identical by descent (from the same ancestor).
• Imagine a population where a fraction (1 - F) of gametes come from random mating.
• The remaining fraction (F) comes from inbreeding, leading to more homozygotes.
• Increased inbreeding leads to fewer heterozygotes and more homozygotes
• Equation for Genotype Frequencies in an Inbred Population:
• Homozygotes: p2(1-F)+pF (for A1A1) and q2(1-F)+qF (for A2A2).
• Heterozygotes: 2pq(1-F) (reduced due to inbreeding).

Disassortative Mating (Opposite of Inbreeding)

• Definition: Mating preference for genetically different partners, increasing heterozygosity.
• Example: White-Throated Sparrows
• Two color morphs: white-striped (dominant, aggressive) and tan-striped (smaller, less aggressive).
• Most matings occur between opposite morphs.
• Experiments Showed:
• Females prefer tan-striped males, however dominant white-striped females outcompete tan-striped females.
• This ultimately forces tan-striped females to mate with white-striped males, leading to disassortative mating.

Summary of Key Takeaways

• Inbreeding depression reduces fitness due to increased homozygosity.
• The main cause of inbreeding depression is the expression of deleterious recessive alleles.
• Small and isolated populations are at greater risk of inbreeding depression.
• F-statistics help to quantify inbreeding.
• Disassortative mating helps maintain genetic diversity by favoring heterozygosity.