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Mating Systems

Non-random mating and its effects on genotype frequencies

Try it: Assortative MatingAssortative Matrix

Non-Random Mating

The Hardy-Weinberg principle assumes random mating — individuals pair without regard to genotype. In nature, mating is often non-random, which changes genotype frequencies. Whether it also changes allele frequencies depends on who mates with whom and how many offspring each pairing leaves.

Key insight: Inbreeding and positive assortative mating with equal fitness (every individual mates and leaves the same number of offspring) change genotype frequencies but leave allele frequencies unchanged. Negative assortative mating changes them: on the Assortative Mating page the Negative model moves p from 0.2 to 0.29 in one generation and settles at 0.25. So does any rule under which some crosses leave no offspring: the Sterile variant on the Assortative Matrix page takes p from 0.2 to 0.002 in four generations. These cases act as selection on mating success.

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 = (Hexp - Hobs) / Hexp 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. A fraction α of individuals mate with their own genotype and the rest mate at random, so H′ = αH/2 + (1 − α)·2pq. Heterozygosity settles at the equilibrium Ĥ = 2pq(1 − α)/(1 − α/2), which stays above zero for α < 1. At α = 0, mating is random (HWE maintained). At α = 1, only like genotypes mate and H halves every generation.

0.30
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.