Wikipedia: Sex

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Many species of living things exist in two or more forms that combine genetic material in order to reproduce. This is called sexual reproduction. Typically, a species will have two forms, called male and female (the female being the one that produces the larger gamete). Fungi and some other organisms exist in more than two sexes, but still reproduce in pairs (any two differing sexes may reproduce). The word sex is also used to refer to sexual intercourse, the physical acts related to sexual reproduction. Some species are capable of both sexual and asexual reproduction.

In animals, sex is determined by a special sex chromosomes, whose alleles are called X and Y. Males typically have one of each, while females typically have two X chromosomes. Since all individuals have at least one X, the Y chromosome is generally reduced and occasionally absent. In deuterostomes (people and other vertebrates, echinoderms, etc) it is the deciding factor in sex determination, but in protostomes (insects, worms, molluscs, etc) the second X chromosome plays that role, resulting in different sexes for combinations like XO.

Increasingly, the term sex is distinguished from the term gender?. Sex is used to refer to the biological division into male and female, while gender is used to the division as a social and cultural construction. There is an increasing awareness that the two sexes are not discrete, but rather a continuum, along which lie many intermediate positions (see intersexuals). Many argue that the division of human beings into male and female is a social construction, and that in reality there are at least five sexes (male, female, merm, ferm and herm), although the first two are the most common.