What is the complementary sex determiner and why does it matter? Sex in honey bees is determined by a gene named the complementary sex determiner (csd). Unfertilised eggs emerge to become drones, whilst fertilised eggs become queen and worker bees. Like other genes, csd has numerous alleles. Alleles are different variations of the same gene. A queen bee has two chromosomes, each carrying a different csd allele. As drones emerge from unfertilised eggs, they only carry one csd allele from their queen mother. Worker bees will carry one of their queen mother’s allele and a different allele from their drone father.  With this system, it is possible that some fertilised eggs end up with the same csd allele from both [...]