Hermaphroditic nematodes can self-fertilize; the offspring are mostly hermaphrodites; less than 1% are males. But when hermaphrodites are mated with males, the offspring are about 1:1 hermaphrodites:male.

Sex is determined by the ratio of X chromosomes to the number of sets of autosomes: hermaphrodites have 2 X chromosomes and 2 copies of each autosome, while males have only 1 X chromosome.