This article is about sex in sexually reproducing organisms. For the act, see Sexual intercourse. For other uses, see Sex (disambiguation) . Organisms of many species are specialized into male and female varieties, each known as a sex ,[1] with some falling in between being intersex . 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. 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 ). Among humans and other mammals , males typically carry XY chromosomes , whereas females typically carry XX chromosomes, which are a part of the XY sex-determination system . Other animals have a sex-determination system as well, such as the ZW sex-determination system in birds, and the X0 sex-determination system in insects. 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. For instance, mate choice and sexual selection can accelerate the evolution of physical differences between the sexes. Overview The male gamete ( sperm) fertilizing the female gamete ( ovum ) 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. Initially the reproduction was a replicating process that consists in producing new individuals that contain the same genetic information as the original or parent individual. This mode of reproduction is called asexual , and it is still used by many species, particularly unicellular, but it is also very common in multicellular organisms. [2] In sexual reproduction, the genetic material of the offspring comes from two different individuals. As sexual reproduction developed by way of a long process of evolution, intermediates exist. Bacteria, for instance, reproduce asexually, but undergo a process by which a part of the genetic material of an individual (donor) is transferred to an other (recipient). [3] Disregarding intermediates, the basic distinction between asexual and sexual reproduction is the way in which the genetic material is processed. Typically, prior to an asexual division, a cell duplicates its genetic information content, and then divides. This process of cell division is called mitosis . 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. [4] 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). [5] In complex organisms, the sex organs are the parts that are involved in the production and exchange of gametes in sexual reproduction. Many species, particularly animals, have sexual specialization, and their populations are divided into male and female individuals. Conversely, there are also species in which there is no sexual specialization, and the same individuals both contain masculine and feminine reproductive organs, and they are called hermaphrodites. This is very frequent in plants. [6] Evolution Main article: Evolution of sexual reproduction Different forms of anisogamy : A) anisogamy of motile cells, B) oogamy (egg cell and sperm cell), C) anisogamy of non-motile cells (egg cell and spermatia). Different forms of isogamy: A) isogamy of motile cells , B) isogamy of non-motile cells, C) conjugation. Sexual reproduction first probably evolved about a billion years ago within ancestral single-celled eukaryotes . [7] The reason for the evolution of sex, and the reason(s) it has survived to the present, are still matters of debate. Some of the many plausible theories include: that sex creates variation among offspring, sex helps in the spread of advantageous traits, that sex helps in the removal of disadvantageous traits, and that sex facilitates repair of germ-line DNA. Sexual reproduction is a process specific to eukaryotes , organisms whose cells contain a nucleus and mitochondria. In addition to animals,

• published: 08 Oct 2017
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