Honey bees are haplodiploid: unfertilised eggs (one set of chromosomes) become haploid males (drones), while fertilised eggs (two sets) become diploid females (workers or queens). Sex is refined by a single complementary sex determiner (csd) gene. A fertilised egg only develops as a normal female if its two csd alleles differ. If, by chance, both copies match, the result is a rare, non-viable diploid drone — nurse bees detect the abnormal pheromone signature and eat the larva shortly after hatching, leaving the classic "shot" or "pepper-pot" gap in the brood pattern.
2 / n, where n is the number of csd alleles circulating in the wider population — fewer alleles (small, isolated, or inbred populations) means more diploid drones.Because diploid drone larvae are eaten within hours of hatching, a patchy "pepper-pot" brood pattern is one of the clearest field signs that a population's csd allele pool has become dangerously narrow — a key reason breeders exchange queen stock between distant apiaries rather than always requeening from their own line.
A queen bee mates high above the comb with a ring of drones, each a distinct patriline. As she lays, the comb fills cell by cell — most eggs become healthy diploid females, but eggs that happen to inherit two matching csd alleles become non-viable diploid drones, spotted and removed by a nurse bee.
Sex in honey bees is set by ploidy first (haploid drones vs. diploid females) and then refined by the csd locus: a diploid egg homozygous at csd develops into a doomed diploid drone. Fewer csd alleles in the wider population sharply raise this risk.
Adjust the queen's mating number and the csd allele diversity in the population, then watch the comb fill with colour-coded patrilines while the diploid-drone rate, genetic diversity index and colony resilience update live.
A patchy "pepper-pot" brood pattern in a real hive is one of the clearest field signs of a shrinking csd allele pool — a key reason breeders exchange queen stock between distant, unrelated apiaries.