https://en.wikipedia.org/wiki/Sex her uses, see Sex (disambiguation). 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.[1][2] The type of gametesproduced 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).[3] Organisms that produce both types of gametes are called hermaphrodites.[2][4] 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. Males and females of a species may be similar (sexual monomorphism), 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 (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,[5] may be referred to as mating types.[6] 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). 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.[7] Sexual reproduction Main article: Sexual reproduction Further information: Isogamy and Anisogamy The life cycle of sexually reproducing organisms cycles through haploid and diploid stages 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,[8] 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.[9] The combination of chromosomal crossover and fertilization, bringing the two single sets of chromosomes together to make a new diploid zygote, results in new organisms that contain different sets of the genetic traits of each parent. In animals, the haploid stage only occurs in the gametes, the haploid cells that are specialized to fuse to form a zygote that develops into a new diploid organism. In plants, the diploid organism produces haploid spores by meiosis that are capable of undergoing repeated cell division to produce multicellular haploid organisms. In either case, gametes may be externally similar (isogamy) as in the green alga Ulva or may be different in size and other aspects (anisogamy).[10] The size difference is greatest in oogamy, a type of anisogamy in which a small, motile gamete combines with a much larger, non-motile gamete.[11] By convention, the larger gamete (called an ovum, or egg cell) is considered female, while the smaller gamete (called a spermatozoon, or sperm cell) is considered male. An individual that produces exclusively large gametes is female, and one that produces exclusively small gametes is male.[12] An individual that produces both types of gametes is a hermaphrodite. In some cases hermaphrodites are able to self-fertilize and produce offspring on their own, without a second Animals
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