Sex in the honeybee is determined by the allelic composition of a single gene, the complementary sex determiner (csd)19. Eggs develop into males when csd is hemizygous (haploid) or homozygous, or females when csd is heterozygous. Honeybees lack sex chromosomes and X-specific dosage compensation. Sex-specific information is transferred in both species from diverged initial signals to the final gene, dsx, via switch genes that are active (on) in the females, but inactive (off) in the males. Most Drosophila pathway genes are present in the honeybee genome despite the marked differences (see text).