Honey bees have no X or Y chromosome. Sex is set by ploidy: a fertilized egg receives a sperm nucleus and develops as a diploid female (2n = 32 chromosomes); an unfertilized egg develops as a haploid male (n = 16), a process called arrhenotoky. The queen controls fertilization by releasing or withholding stored sperm as each egg passes through her oviduct.
Ploidy alone is not the whole story. Real femaleness needs heterozygosity at a single master gene, complementary sex determiner (csd). A diploid egg that happens to inherit the same csd allele from both parents develops into a diploid drone — sterile, and eaten by nurse workers within hours of hatching, wasting the resources the queen invested in it. This is the evolutionary cost that keeps csd allele diversity high in wild populations.
Single-locus CSD model, egg by egg:
egg fertilized with probability p
-> diploid (2n)
draw maternal allele a, paternal allele b from N csd alleles
a != b -> HETEROZYGOUS -> female (worker / queen)
a == b -> HOMOZYGOUS -> diploid male (culled by workers)
egg unfertilized with probability (1-p)
-> haploid (n), single csd allele, always hemizygous -> viable male
P(diploid egg is homozygous) ~= 1 / N
P(any egg becomes a doomed diploid drone) ~= p * (1/N)
- Fertilization rate — the queen's sperm-release probability per egg; shifts the diploid:haploid ratio of the whole comb.
- csd alleles (N) — how many distinct csd variants segregate in the population; more alleles make matching, and therefore diploid drones, rarer.
- Egg-laying rate — how fast the queen moves cell to cell; speeds up or slows down the simulation.
- Cull diploid drones — toggles the real worker behaviour of detecting and eating homozygous larvae before they mature.
Because small, inbred, or bottlenecked populations run low on csd alleles, N drops and the diploid-drone tax rises sharply — a real conservation concern for isolated honey bee populations.