Sex is the trait that determines whether a sexually reproducing animal or plant produces male or female gametes. Male plants and animals produce small mobile gametes (spermatozoa, sperm, pollen), while females produce larger, non-motile ones (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), whereas 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. Sexual reproduction Sexual reproduction is a process exclusive to eukaryotes in which organisms produce offspring that possess a selection of the genetic traits of each parent. Genetic traits are contained within the deoxyribonucleic acid (DNA) of chromosomes. The cells of eukaryotes have a set of paired homologous chromosomes, one from each parent, and this double-chromosome stage is called "diploid". During sexual reproduction, diploid organisms produce specialized haploid sex cells called gametes via meiosis, each of which has a single set of chromosomes. Meiosis involves a stage of genetic recombination via chromosomal crossover, in which regions of DNA are exchanged between matched pairs of chromosomes, to form new chromosomes each with a new and unique combination of the genes of the parents. Then the chromosomes are separated into single sets in the gametes. Each gamete in the offspring thus has half of the genetic material of the mother and half of the father. The combination of chromosomal crossover and fertilization, bringing the two single sets