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See also Human sexuality

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 mammals, birds, and many other species, sex is determined by the sex chromosomes, whose alleles are called X and Y. Males typically have one of each (XY), while females typically have two X chromosomes (XX). Since all individuals have at least one X, the Y chromosome is generally reduced, and is absent in some forms, this pattern admitting some considerable variation. In other forms, sex may be determined by environmental factors like temperature, or by age. Many animals are hermaphroditic - that is, individuals may have both male and female parts.

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). Some have argued 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.

See also: Evolution, Gender and sexuality studies