Sex

Sex is a trait that determines an individual's reproductive function, male or female, in animals and plants that propagate their species through sexual reproduction. The type of gametes produced by an organism defines its sex. Commonly in plants and animals, male organisms produce smaller gametes (spermatozoa, sperm) while female organisms produce larger gametes (ova, often called egg cells). Organisms that produce both types of gametes are called hermaphrodites. During sexual reproduction, male and female gametes fuse to form zygotes that develop into offspring that inherit a selection of the traits of each parent. Male and female individuals of a species may be similar, or have physical differences (sexual dimorphism). The differences reflect the different reproductive pressures the sexes experience. For instance, mate choice and sexual selection can accelerate the evolution of physical differences between the sexes. The terms "male" and "female" typically do not apply in sexually undifferentiated species in which the individuals are isomorphic and the gametes are isogamous (indistinguishable in size and morphology), such as the green alga Ulva lactuca. If there are instead functional differences between gametes, such as with fungi, they may be referred to as mating types.Sex is genetically determined in most mammals by the XY sex-determination system, where male mammals carry an X and a Y chromosome (XY), whereas female mammals carry two X chromosomes (XX).