Honey bees don't have X and Y sex chromosomes. Instead, sex is decided by chromosome number. A queen stores sperm from her mating flights and controls, cell by cell, whether an egg is fertilized as she lays it. Unfertilized eggs develop into haploid males (drones, one chromosome set). Fertilized eggs are diploid (two chromosome sets) and normally develop into females — workers or queens, depending on larval diet.
complementary sex determiner. A fertilized egg only develops as a female if its two csd alleles are different (heterozygous).Diploid drones are not a rare curiosity — in a small, closed population they can make up 30% or more of fertilized eggs, which is one reason beekeepers and conservationists worry about inbreeding depression in isolated honey bee populations.
A queen bee lays eggs into a honeycomb, and the chromosome count of each egg — not an X or Y chromosome — decides whether it becomes a drone, a worker, or an unviable diploid drone.
Unfertilized eggs are haploid and become drones; fertilized eggs are diploid and normally become females — unless both copies of the csd gene happen to match, in which case the larva develops as a diploid drone and is removed by workers.
Adjust the fertilization rate and the number of csd alleles circulating in the colony, then watch the comb fill with color-coded eggs and the karyotype panel update with each new egg laid.
In small or inbred honey bee populations with few csd alleles, diploid-drone eggs can make up a large share of fertilized eggs — a major reason breeders track genetic diversity carefully.