Honey bees are haplodiploid: unfertilised eggs (one chromosome set) become haploid drones, fertilised eggs (two sets) become diploid females. A single complementary sex determiner (csd) gene refines this — a fertilised egg only becomes a normal female if its two csd alleles differ. A homozygous match produces a non-viable diploid drone, eaten by nurse bees within hours.
- A queen mates with several drones (patrilines), each a distinct comb colour here.
- P(homozygous) ≈ 2 / n, where n is the number of csd alleles circulating in the population — fewer alleles means more diploid-drone loss.
- More patrilines raise the colony's internal genetic diversity (Simpson index), which improves disease resistance and thermoregulation.
- Resilience combines low diploid loss with high diversity.