PopGeneJS

Non-Random Mating

The Hardy-Weinberg principle assumes random mating — individuals pair without regard to genotype. In nature, mating is often non-random, which affects genotype frequencies without changing allele frequencies.

Key insight: Non-random mating changes genotype frequencies but does NOT change allele frequencies (unless combined with selection).

Types of Non-Random Mating

Assortative Mating

Positive assortative mating: Like pairs with like (similar phenotypes mate more often than expected by chance). This increases homozygosity.

Negative assortative mating (disassortative): Unlike pairs with unlike (dissimilar phenotypes mate more often). This increases heterozygosity.

Inbreeding

Mating between relatives increases the probability that offspring receive identical alleles from a common ancestor. The inbreeding coefficient F measures this:

F = (H_exp - H_obs) / H_exp Deviation from HWE heterozygosity

Effects on Genotype Frequencies

Under inbreeding or positive assortative mating, genotype frequencies shift from HWE:

P(AA) = p² + Fpq Homozygote excess

P(Aa) = 2pq(1-F) Heterozygote deficit

P(aa) = q² + Fpq Homozygote excess

Interactive: Assortative Mating Effects

Adjust the degree of positive assortative mating (α) to see how heterozygosity declines over generations. At α = 0, mating is random (HWE maintained). At α = 1, only like genotypes mate.

Assortment (α) 0.30

Initial p 0.50

Positive assortative mating reduces heterozygosity while preserving allele frequencies.

Inbreeding Depression

Increased homozygosity from inbreeding often reduces fitness because:

Deleterious recessives: Recessive harmful alleles become exposed in homozygotes
Overdominance: If heterozygotes are fittest, homozygotes have lower fitness
Reduced variation: Less genetic diversity for response to environmental change

This inbreeding depression is stronger for fitness-related traits and in populations that are normally outbreeding.

Selfing

Self-fertilization is the extreme of inbreeding (F approaches 1). After t generations of selfing:

H(t) = H₀ × (1/2)^t Halving of heterozygosity each generation

Heterozygosity halves each generation under complete selfing. After 10 generations, only ~0.1% of the original heterozygosity remains.

Outbreeding

When inbred populations cross, the F1 generation shows heterosis (hybrid vigor):

Dominance hypothesis: Heterozygotes mask deleterious recessives
Overdominance hypothesis: Heterozygotes are intrinsically superior

However, crossing very divergent populations can cause outbreeding depression if locally adapted gene combinations are broken up.

Practical Implications

Conservation: Small populations risk inbreeding depression
Agriculture: Inbred lines crossed for hybrid vigor
Human genetics: Consanguineous marriages increase recessive disease risk
Forensics: Population structure affects DNA match probabilities

The effective number of breeders and mating system determine how quickly genetic variation is lost. Managing mating patterns is crucial for conservation of endangered species.