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A haplodiploid species is one in which one of the sexes has haploid cells (cells containing one copy of each chromosome) and the other has diploid cells (cells containing two copies of each chromosome). Most commonly, the male is haploid and the female is diploid. In such species, the male develops from unfertilized eggs, while the female develops from fertilized eggs: the sperm provides a second set of chromosomes when it fertilizes the egg.

Haplodiploidy is found in many species of insects, particularly ants, bees, and wasps. It increases the significance of kin selection, which may explain the eusociality of these sorts of insects.

See also: ploidy