The life cycle of sexually reproducing organisms cycles through haploid and diploid stages Various environmental systems include temperature-dependent sex determination in reptiles and crustaceans. Other chromosomal sex-determination systems in animals include the ZW system in birds, and the X0 system in insects. Most mammals have the XY sex-determination system, where male mammals usually carry an X and a Y chromosome (XY), whereas female mammals usually carry two X chromosomes (XX). The sex of a living organism is determined by its genes. Some kinds of functional differences between gametes, such as in fungi, may be referred to as mating types. The terms male and female typically do not apply in sexually undifferentiated species in which the individuals are isomorphic (look the same) and the gametes are isogamous (indistinguishable in size and shape), such as the green alga Ulva lactuca. Mate choice and sexual selection can accelerate the evolution of physical differences between the sexes. Males and females of a species may have physical similarities (sexual monomorphism) or differences ( sexual dimorphism) that reflect various reproductive pressures on the respective sexes. During sexual reproduction, male and female gametes fuse to form zygotes, which develop into offspring that inherit traits from each parent. Organisms that produce both types of gametes are called hermaphrodites.

Male plants and animals produce small mobile gametes (spermatozoa, sperm, pollen), while females produce larger, non-motile ones ( ova, often called egg cells). Sex is the trait that determines whether a sexually reproducing organism produces male or female gametes.

For other uses, see Sex (disambiguation). This article is about the distinguishing trait in sexually reproducing organisms.

Se woodworking supplies