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The gametes produced by an organism are determined by its sex: males produce male gametes (spermatozoa, or sperm, in animals; pollen in plants) while females produce female gametes (ova, or egg cells); individual organisms which produce both male and female gametes are termed hermaphroditic.

Frequently, physical differences are associated with the different sexes of an organism; these sexual dimorphisms can reflect the different reproductive pressures the sexes experience.

XY sex determination is used by most mammals, and from a comparison between chicken and human, the Z chromosome appeared similar to the autosomal chromosome 9 in human, rather than X or Y, suggesting that the ZW and XY sex-determination systems do not share an origin, but that the sex chromosomes are derived from autosomal chromosomes of the common ancestor of birds and mammals.

A paper from 2004 compared the chicken Z chromosome with platypus X chromosomes and suggested that the two systems are related.

For instance, mate choice and sexual selection can accelerate the evolution of physical differences between the sexes.

One of the basic properties of life is reproduction, the capacity to generate new individuals, and sex is an aspect of this process.

The resulting cells are called gametes, and contain only half the genetic material of the parent cells.

These gametes are the cells that are prepared for the sexual reproduction of the organism.

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Gametes can be identical in form and function (known as isogamy), but in many cases an asymmetry has evolved such that two sex-specific types of gametes (heterogametes) exist (known as anisogamy).Multiplicity of gamete types within a species would still be considered a form of sexual reproduction.However, no third gamete is known in multicellular animals.This double-chromosome stage is called "diploid", while the single-chromosome stage is "haploid".Diploid organisms can, in turn, form haploid cells (gametes) that randomly contain one of each of the chromosome pairs, via meiosis.

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