Sex

Sex is the biological trait of a reproducing organism in producing gametes of one of two different sizes or shapes—male or female gametes. Thus, the typical classification for such organisms, their sex, is also known as either male or female. During sexual reproduction, a male and a female gamete fuse to form a zygote, which develops into an offspring that inherits traits from each parent. By convention, organisms that produce smaller gametes (spermatozoa, sperm) are called male, while organisms that produce larger gametes (ova, often called egg cells) are called female. An organism that produces both types of gamete is a hermaphrodite.

In non-hermaphroditic species, the sex of an individual is determined through one of several biological sex-determination systems. Most mammalian species have the XY sex-determination system, where the male usually carries an X and a Y chromosome (XY), and the female usually carries two X chromosomes (XX). Other chromosomal sex-determination systems in animals include the ZW system in birds and the XO system in some insects. Various environmental systems include temperature-dependent sex determination in reptiles and crustaceans.