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Haplodiploidy is a sex-determination system in which males develop from unfertilized eggs and are haploid (n) and females develop from fertilized eggs and are diploid (2n). Haplodiploidy determines the sex in all members of the insect order Hymenoptera (bees, ants, and wasps).

The haplodiploid sex-determination system has a number of peculiarities. For example, a male has no father and cannot have sons (but he has a grandfather and can have grandsons). Additionally, recessive lethal and deleterious alleles will be removed from the population rapidly because they will automatically be expressed in the males. [In other words, dominant lethal and deleterious alleles are removed from the population every time they arise as they kill any individual in which they occur.]

Haplodiploidy is not the same thing as an XO sex-determination system. In Haplodiploidy, males receive one half of the chromosomes that females receive (including autosomes). In an XO sex-determination system, males and females receive an equal number of autosomes. However, when it comes to sex chromosomes, females receive two X-chromosomes while males receive only a single X-chromosome.

In honeybees, drones (males) are entirely derived from the queen (e.g. their mother). The diploid queen has 32-chromosomes and the haploid drones have 16-chromosomes. Drones produce sperm cells that contain their entire genome. Thus, the sperm are all genetically identical (except for mutations). The male bees' genetic makeup is therefore entirely derived from the queen while the genetic makeup of female worker bees is half derived from the queen and half from the father. If a queen bee mates with only one drone, any two of her daughters will share, on average, 75% of their genes. Finally, the diploid queen's genome is recombined for her daughters. However, the haploid father's genome is inherited by his daughters "as is." The natural world never ceases to amaze.