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.