These gametes combine to form embryos which develop into new organisms.

The gametes of animals have male and female forms- spermatozoa and egg cells. Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes. In some cases hermaphrodites are able to self-fertilize and produce offspring on their own, without a second An individual that produces both types of gametes is a hermaphrodite. An individual that produces exclusively large gametes is female, and one that produces exclusively small gametes is male. Д«y 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. 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. 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). In plants, the diploid organism produces haploid spores by meiosis that are capable of undergoing repeated cell division to produce multicellular haploid organisms. 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. 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. Each gamete in the offspring thus has half of the genetic material of the mother and half of the father. Then the chromosomes are separated into single sets in the gametes. 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. During sexual reproduction, diploid organisms produce specialized haploid sex cells called gametes via meiosis, each of which has a single set 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'. Genetic traits are contained within the deoxyribonucleic acid (DNA) of chromosomes. Sexual reproduction is a process exclusive to eukaryotes in which organisms produce offspring that possess a selection of the genetic traits of each parent. 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 carry an X and a Y chromosome (XY), whereas female mammals carry two X chromosomes (XX). ДЄ living thing's sex 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 smaller gametes (spermatozoa, sperm) while females produce larger ones ( ova, often called egg cells). Sex is the trait that determines whether a sexually reproducing animal or plant produces male gametes or female ones. For other uses, see Sex (disambiguation). This article is about sex in sexually reproducing organisms.

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