Sex is the biological trait that determines whether a sexually reproducing organism produces male or female gametes. Male organisms produce small mobile gametes (spermatozoa, sperm, pollen), while female organisms produce larger, non-mobile 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, which develop into offspring that inherit traits from each parent. 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. 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 (look the same) and the gametes are isogamous (indistinguishable in size and shape), such as the green alga Ulva lactuca. Some kinds of functional differences between gametes, such as in fungi, may be referred to as mating types.The sex of a living organism is determined by its genes. Most mammals have the XY sex-determination system, where male mammals usually carry an X and a Y chromosome (XY), and female mammals usually carry two X chromosomes (XX). Other chromosomal sex-determination systems in animals include the ZW system in birds, and the X0 system in insects. Various environmental systems include temperature-dependent sex determination in reptiles and crustaceans.