Meiosis also involves a stage of chromosomal crossover in which regions of DNA are exchanged between matched types of chromosomes, to form new pairs of mixed chromosomes, each of which is a blend of the genes of both parents. Diploid organisms can, in turn, produce haploid cells ( gametes) that randomly contain one of each of the chromosome pairs, via meiosis. This double-chromosome stage is called ' diploid' while the single-chromosome stage is ' haploid'. By combining one set of chromosomes from each parent, an organism is formed containing a double set of chromosomes. The codes for genetic traits are contained within the deoxyribonucleic acid (DNA) of chromosomes. Sexual reproduction is production of offspring by the fusion of haploid gametes. 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.

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). If there are instead functional differences between gametes, such as in fungi, they 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. For instance, mate choice and sexual selection can accelerate the evolution of physical differences between the sexes.

The differences reflect the different reproductive pressures the sexes experience. Males and females of a species may be similar (sexual monomorphism), or have physical differences ( sexual dimorphism).

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. Organisms that produce both types of gametes are called hermaphrodites. Commonly in plants and animals, male organisms produce smaller gametes (spermatozoa, sperm) while female organisms produce larger gametes ( ova, often called egg cells). The type of gametes produced by an organism define its sex. 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. For other uses, see Sex (disambiguation). This article is about sex in sexually reproducing organisms.

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