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The ZW sex-determination system is shared by birds, some fish and some crustaceans.

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.

Sexual reproduction involves the combining and mixing of genetic traits: specialized cells known as gametes combine to form offspring that inherit traits from each parent.

The gametes produced by an organism define its sex: males produce small gametes (e.g.

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

Life has evolved from simple stages to more complex ones, and so have the reproduction mechanisms.

When it comes to activities, most peope think first of water-borne activities.Typically, prior to an asexual division, a cell duplicates its genetic information content, and then divides. In sexual reproduction, there are special kinds of cells that divide without prior duplication of its genetic material, in a process named meiosis.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.Sex comprises the arrangements that enable sexual reproduction, and has evolved alongside the reproduction system, starting with similar gametes (isogamy) and progressing to systems that have different gamete types, such as those involving a large female gamete (ovum) and a small male gamete (sperm).Bacteria, for instance, reproduce asexually, but undergo a process by which a part of the genetic material of an individual donor is transferred to another recipient.

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