Sex refers to the male and female duality of biology and reproduction, a process in biological DNA that dates back 4.6 - 3.5 billion years. DNA links back in an unbroken series of sexual reproduction taking forward information to present day. The somewhat similar term gender has more to do with identity than biology. The concept is confined to organisms that reproduce sexually.

The female sex is definitely defined as the one which produces the larger gamete (i.e., reproductive cell) and which typically bears the offspring. The category of sex reflects the biological reproductive function, rather than sexuality or other behaviors. In some animals, sex may be assigned to specific structures rather than the entire organism as some species, such as earthworms, are normally hermaphroditic.

Sex in non-animal species