Editing Sex (section)

==Sexual reproduction==
{{Main|Sexual reproduction}}
{{Further|Isogamy|Anisogamy}}
[[File:Sexual cycle.svg|thumb|left|The life cycle of a sexually reproducing species cycles through haploid and diploid stages]]

Sexual reproduction, in which two individuals produce an offspring that possesses a selection of the genetic traits of each parent, is exclusive to [[eukaryote]]s. Genetic traits are encoded in the [[DNA|deoxyribonucleic acid]] (DNA) of [[chromosome]]s. The eukaryote cell has a set of paired [[homologous chromosomes]], one from each parent, and this double-chromosome stage is called "[[diploid]]". During sexual reproduction, a diploid organism produces specialized [[haploid]] sex cells called [[gametes]] via [[meiosis]],<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|location=New York | chapter = Meiosis | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK26840/ | archive-url = https://web.archive.org/web/20170125115052/https://www.ncbi.nlm.nih.gov/books/NBK26840/ | archive-date=25 January 2017}}</ref> each of which has a single set of chromosomes. 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 combination of the genes of the parents. Then the chromosomes are separated into single sets in the gametes. When gametes fuse during fertilization, the resulting zygote has half of the genetic material of the mother and half of the father.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|location=New York | chapter = The Benefits of Sex | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3678 }}</ref> The combination of chromosomal crossover and [[fertilization]], bringing the two single sets of chromosomes together to make a new diploid [[zygote]], results in a new organism that contains a different set of the genetic traits of each parent.

In [[animal]]s, the haploid stage only occurs in the gametes, the sex cells that fuse to form a zygote that develops directly into a new diploid organism. In a [[plant]] species, the diploid organism produces a type of haploid [[spore]] by meiosis that is capable of undergoing repeated [[mitosis|cell division]] to produce a [[multicellular]] haploid organism. In either case, the gametes may be externally similar ([[isogamy]]) as in the green alga ''Ulva'' or may be different in size and other aspects ([[anisogamy]]).<ref>{{cite book |title=Developmental Biology |vauthors=[[Scott F. Gilbert | Gilbert SF]] |date=2000 |publisher=Sinauer Associates |isbn=978-0-87893-243-6 |edition=6th |location=Sunderland (MA) |chapter=Multicellularity: Evolution of Differentiation |access-date=17 April 2021 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10031 |archive-url=https://web.archive.org/web/20210308143406/https://www.ncbi.nlm.nih.gov/books/NBK10031/ |archive-date=8 March 2021 |url-status=live}}</ref>  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.<ref>{{cite book|url={{GBurl|id=wzZGQOmcjqAC|q=a%20dictionary%20of%20plant%20sciences%20oogamy|p=350}}|title=A Dictionary of Plant Sciences|vauthors=Allaby M|date=2012|publisher=OUP Oxford|isbn=978-0-19-960057-1|page=350|language=en|author-link=Michael Allaby}}</ref>

In anisogamic organisms, by 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. An individual that produces large gametes is female, and one that produces small gametes is male.<ref>{{cite news | vauthors = Gee H |author-link=Henry Gee |date=22 November 1999 |title=Size and the single sex cell |url=https://www.nature.com/news/1999/991122/full/news991125-4.html |url-status=live |archive-url=https://web.archive.org/web/20171011062235/http://www.nature.com/news/1999/991122/full/news991125-4.html |archive-date=11 October 2017 |access-date=4 June 2018 |work=Nature |ref=10.1038/news991125-4}}</ref> An individual that produces both types of gamete is a [[hermaphrodite]]. In some species, a hermaphrodite can [[self-fertilization|self-fertilize]] and produce an offspring on its own.             

===Animals===
{{Main|Sexual reproduction#Animals}}
[[File:Hoverflies mating midair.jpg|thumb|left|''[[Simosyrphus grandicornis]]'' mating]]

Most sexually reproducing animals spend their lives as diploid, with the haploid stage reduced to single-cell gametes.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|location=New York | chapter = Mendelian genetics in eukaryotic life cycles | chapter-url = https://www.ncbi.nlm.nih.gov/books/NBK21836 | archive-url = https://web.archive.org/web/20170402185423/https://www.ncbi.nlm.nih.gov/books/NBK21836/ | archive-date = 2 April 2017}}</ref> The gametes of animals have male and female forms—[[spermatozoa]] and egg cells, respectively. These gametes combine to form [[embryos]] which develop into new organisms.

The male gamete, a [[spermatozoon]] (produced in vertebrates within the [[testes]]), is a small cell containing a single long [[flagellum]] which propels it.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|location=New York | chapter = Sperm | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3729 | archive-url = https://web.archive.org/web/20090629222617/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3729 | archive-date= 29 June 2009 }}</ref> Spermatozoa are extremely reduced cells, lacking many cellular components that would be necessary for embryonic development. They are specialized for motility, seeking out an egg cell and fusing with it in a process called [[Fertilization#Fertilisation in animals|fertilization]].

Female gametes are egg cells. In vertebrates, they are produced within the [[ovary|ovaries]]. They are large, immobile cells that contain the nutrients and cellular components necessary for a developing embryo.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|location=New York | chapter = Eggs | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3718 | archive-url = https://web.archive.org/web/20090629074430/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3718 | archive-date= 29 June 2009}}</ref> Egg cells are often associated with other cells which support the development of the embryo, forming an [[Egg (biology)|egg]]. In mammals, the fertilized embryo instead develops within the female, receiving nutrition directly from its mother.

Animals are usually<!--h'm. but not Sponges, Corals, Bryozoa, Barnacles, ...--> mobile and seek out a partner of the opposite sex for [[mating]]. Animals which live in the water can mate using [[external fertilization]], where the eggs and sperm are released into and combine within the surrounding water.<ref>{{cite book|vauthors=[[Bruce Alberts|Alberts B]], [[Alexander D. Johnson|Johnson A]], [[Julian Lewis (biologist)|Lewis J]], [[Martin Raff|Raff M]], Roberts K, [[Peter Walter|Walter P]]|title=Molecular Biology of the Cell|edition=4th|year=2002|isbn=978-0-8153-3218-3|publisher=Garland Science|location=New York | chapter = Fertilization | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3738 | archive-url = https://web.archive.org/web/20081219005819/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mboc4.section.3738 | archive-date= 19 December 2008 }}</ref> Most animals that live outside of water, however, use [[internal fertilization]], transferring sperm directly into the female to prevent the gametes from drying up.<!--Dragonflies use indirect method, transferring a spermatheca.-->

In most birds, both excretion and reproduction are done through a single posterior opening, called the [[cloaca]]—male and female birds touch cloaca to transfer sperm, a process called "cloacal kissing".<ref>{{cite web |title=Avian Reproduction |url=http://people.eku.edu/ritchisong/avianreproduction.html |publisher=Eastern Kentucky University | vauthors = Ritchison G |access-date=3 April 2008 |archive-date=12 April 2008 |archive-url=https://web.archive.org/web/20080412231002/http://people.eku.edu/ritchisong/avianreproduction.html |url-status=live }}</ref> In many other terrestrial animals, males use specialized [[sex organ]]s to assist the transport of sperm—these male sex organs are called [[intromittent organ]]s. In humans and other mammals, this male organ is known as the [[penis]], which enters the female reproductive tract (called the [[vagina]]) to achieve [[insemination]]—a process called [[sexual intercourse]]. The penis contains a tube through which [[semen]] (a fluid containing sperm) travels. In female mammals, the vagina connects with the [[uterus]], an organ which directly supports the development of a fertilized embryo within (a process called [[gestation]]).

Because of their motility, [[animal sexual behavior]] can involve coercive sex. [[Traumatic insemination]], for example, is used by some insect species to inseminate females through a wound in the abdominal cavity—a process detrimental to the female's health.

===Plants===
{{Main|Plant reproduction}}
[[File:Mature flower diagram.svg|thumb|left|Flowers contain the sexual organs of flowering plants. They are usually hermaphrodite, containing both male and female parts.]]
Like animals, [[land plants]] have specialized male and female gametes.<ref>{{cite book |title=Developmental Biology |vauthors=[[ Scott F. Gilbert | Gilbert SF]] |date=2000 |publisher=Sinauer Associates |isbn=978-0-87893-243-6 |edition=6th |location=Sunderland (MA) |chapter=Gamete Production in Angiosperms |access-date=17 April 2021 |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK10129/ |archive-url=https://web.archive.org/web/20210421143911/https://www.ncbi.nlm.nih.gov/books/NBK10129/ |archive-date=21 April 2021 |url-status=live}}</ref><ref name="Dusenbery-2009">{{cite book|url={{GBurl|id=QCrimQJu1RAC|q=living+at+micro+scale+reproduction|p=308}}|title=Living at Micro Scale: The Unexpected Physics of Being Small|vauthors=Dusenbery DB|date=2009|publisher=Harvard University Press|isbn=978-0-674-03116-6|pages=308–326|language=en|author-link=David B. Dusenbery}}</ref> In [[Spermatophyte|seed plants]], male gametes are produced by reduced male [[gametophytes]] that are contained within [[pollen]] which have hard coats that protect the male gamete forming cells during transport from the [[Stamen|anther]]s to the [[Stigma (botany)|stigma]]. The female gametes of seed plants are contained within [[ovule]]s. Once fertilized, these form [[seed]]s which, like eggs, contain the nutrients necessary for the initial development of the embryonic plant.

{{multiple image
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 | image1 = Pinus nigra cone.jpg
 | image2 = Pine cones, immature male.jpg
 | footer = Female (left) and male (right) cones contain the sex organs of pines and other conifers.
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The [[flower]]s of [[flowering plant]]s contain their sexual organs. Most flowering plants are hermaphroditic, with both male and female parts in the same flower or on the same plant in single sex flowers, about 5% of plant species have individual plants that are one sex or the other.<ref>{{cite web | vauthors = Wilton P | date = 12 March 2009 |title=Plants, sex & Darwin | work = OxSciBlog | publisher = University of Oxford |url=https://www.ox.ac.uk/news/science-blog/plants-sex-darwin |access-date=10 January 2024 |language=en}}</ref> The female parts, in the center of a hermaphroditic or female flower, are the [[pistils]], each unit consisting of a [[carpel]], a [[Style (botany)|style]] and a [[stigma (botany)|stigma]]. Two or more of these reproductive units may be merged to form a single compound [[pistil]], the fused carpels forming an [[Ovary (botany)|ovary]]. Within the carpels are [[ovules]] which develop into seeds after fertilization. The male parts of the flower are the [[stamen]]s: these consist of long filaments arranged between the pistil and the petals that produce pollen in [[Glossary of botanical terms#anther|anthers]] at their tips. When a pollen grain lands upon the stigma on top of a carpel's style, it germinates to produce a [[pollen tube]] that grows down through the tissues of the style into the carpel, where it delivers male gamete nuclei to fertilize an ovule that eventually develops into a seed.

Some hermaphroditic plants are self-fertile, but plants have evolved multiple different [[self-incompatibility]] mechanisms to avoid self-fertilization, involving [[sequential hermaphroditism]], molecular recognition systems and morphological mechanisms such as [[heterostyly]].<ref name="Judd-2002">{{cite book | vauthors =  Judd WS, Campbell CS, Kellogg EA, Stevens PF, Donoghue MJ |author-link1=Walter Stephen Judd |author-link3=Elizabeth Anne Kellogg |author-link4=Peter F. Stevens |author-link5=Michael Donoghue |title=Plant systematics, a phylogenetic approach  |date=2002 |publisher=Sinauer Associates Inc. |isbn=0-87893-403-0 |edition=2nd |location=Sunderland MA}}</ref>{{rp|73, 74}}

In [[pine]]s and other [[conifer]]s, the sex organs are produced within [[conifer cone|cones]] that have male and female forms. Male cones are smaller than female ones and produce pollen, which is transported by wind to land in female cones. The larger and longer-lived female cones are typically more durable, and contain ovules within them that develop into seeds after fertilization.

Because [[Spermatophyte|seed plants]] are immobile, they depend upon passive methods for transporting pollen grains to other plants. Many, including conifers and grasses, produce lightweight pollen which is carried by wind to neighboring plants. Some flowering plants have heavier, sticky pollen that is specialized for transportation by insects or larger animals such as [[hummingbird]]s and [[bat]]s, which may be attracted to flowers containing rewards of nectar and pollen. These animals transport the pollen as they move to other flowers, which also contain female reproductive organs, resulting in [[pollination]].

===Fungi===
{{Main|Mating in fungi}}
[[File:Shiitake mushroom.jpg|thumb|Mushrooms are produced as part of fungal sexual reproduction.]]

Most species of [[fungus]] can reproduce sexually and have life cycles with both haploid and diploid phases. These species of fungus are typically [[isogamy|isogamous]], i.e. lacking male and female specialization. One haploid fungus grows into contact with another, and then they fuse their cells. In some cases, the fusion is asymmetric, and the cell which donates only a nucleus (and no accompanying cellular material) could arguably be considered male.<ref>{{cite book |author=Nick Lane |title=Power, Sex, Suicide: Mitochondria and the Meaning of Life |url=https://archive.org/details/powersexsuicidem0000lane |url-access=registration |pages=[https://archive.org/details/powersexsuicidem0000lane/page/236 236–237] |isbn=978-0-19-280481-5 |year=2005 |publisher=Oxford University Press}}</ref> Fungi may also have more complex allelic mating systems, with other sexes not accurately described as male, female, or hermaphroditic.<ref name="Watkinson-2015">{{cite book|url={{GBurl|id=x8qcBAAAQBAJ|p=115}}|title=The Fungi|vauthors=Watkinson SC, [[Lynne Boddy|Boddy L]], Money N|publisher=Elsevier Science|year=2015|isbn=978-0-12-382035-8|page=115|access-date=18 February 2018}}</ref>

Some fungi, including [[baker's yeast]], have [[mating type]]s that determine compatibility. Yeasts with the same mating types will not fuse with each other to form diploid cells, only with yeast carrying another mating type.<ref>{{cite book | vauthors = Lodish H, Berk A, Zipursky SL, Matsudaira P, Baltimore D, Darnell J |year=2000 |title=Molecular Cell Biology |edition=Fourth |publisher=W.H. Freeman and Co |isbn=978-0-7167-4366-8 | chapter = Cell-Type Specification and Mating-Type Conversion in Yeast | chapter-url = https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.section.3752 |archive-url=https://web.archive.org/web/20090701170903/http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb.section.3752 | archive-date=1 July 2009 }}</ref>

Many species of [[Dikarya|higher fungi]] produce [[mushroom]]s as part of their [[Fungus#Reproduction|sexual reproduction]]. Within the mushroom, diploid cells are formed, later dividing into haploid [[spore]]s.