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The biology of gender became the subject of an expanding number of studies over the course of the late 20th century. One of the earliest areas of interest was what is now called gender identity disorder (GID). Studies in this, and related areas, inform the following summary of the subject by John Money. He stated: The term "gender role" appeared in print first in 1955. The term gender identity was used in a press release, November 21, 1966, to announce the new clinic for transsexuals at The Johns Hopkins Hospital. It was disseminated in the media worldwide, and soon entered the vernacular. The definitions of gender and gender identity vary on a doctrinal basis. In popularized and scientifically debased usage, sex is what you are biologically; gender is what you become socially; gender identity is your own sense or conviction of maleness or femaleness; and gender role is the cultural stereotype of what is masculine and feminine. Causality with respect to gender identity disorder is subdivisible into genetic, prenatal hormonal, postnatal social, and postpubertal hormonal determinants, but there is, as yet, no comprehensive and detailed theory of causality. Gender coding in the brain is bipolar. In gender identity disorder, there is discordancy between the natal sex of one's external genitalia and the brain coding of one's gender as masculine or feminine.[76] Money refers to attempts to distinguish a difference between biological sex and social gender as "scientifically debased", because of our increased knowledge of a continuum of dimorphic features (Money's word is "dipolar") that link biological and behavioral differences. These extend from the exclusively biological "genetic" and "prenatal hormonal" differences between men and women, to "postnatal" features, some of which are social, but others have been shown to result from "postpubertal hormonal" effects. Although causation from the biological—genetic and hormonal—to the behavioural has been broadly demonstrated and accepted, Money is careful to also note that understanding of the causal chains from biology to behaviour in sex and gender issues is very far from complete. For example, the existence of a "gay gene" has not been proven, but such a gene remains an acknowledged possibility.[77] There are studies concerning women who have a diagnosis called congenital adrenal hyperplasia, which leads to the overproduction of masculinizing sex hormones, androgens. These women usually have normal female appearances (though nearly all girls with CAH have corrective surgery performed on their genitals) but despite of hormone-balancing medication that they are given since birth, they are statistically more likely to be interested in activities traditionally linked to males than females. Psychology professor and CAH researcher Dr. Sheri Berenbaum attributes these differences to exposure to higher levels of male sex hormones in utero.[78] Sexual reproduction This section may require cleanup to meet Wikipedia's quality standards. No cleanup reason has been specified. Please help improve this section if you can. (May 2010) Main article: Sexual reproduction Sexual differentiation demands the fusion of gametes that are morphologically different. —Cyril Dean Darlington, Recent Advances in Cytology, 1937. Hoverflies mating Sexual reproduction is a common method of producing a new individual within various species. In sexually reproducing species, individuals produce special kinds of cells (called gametes) whose function is specifically to fuse with one unlike gamete and thereby to form a new individual. This fusion of two unlike gametes is called fertilization. By convention, where one type of gamete cell is physically larger than the other, it is associated with female sex. Thus an individual that produces exclusively large gametes (ova in humans) is called female, and one that produces exclusively small gametes (spermatozoa in humans) is called male. An individual that produces both types of gametes is called hermaphrodite (a name applicable also to people with one testis and one ovary). In some species hermaphrodites can self-fertilize (see Selfing), in others they can achieve fertilization with females, males or both. Some species, like the Japanese Ash, Fraxinus lanuginosa, only have males and hermaphrodites, a rare reproductive system called androdioecy. Gynodioecy is also found in several species. Human hermaphrodites are typically, but not always, infertile. What is considered defining of sexual reproduction is the difference between the gametes and the binary nature of fertilization. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. However, of more than 1.5 million living species,[79] recorded up to about the year 2000, "no third sex cell—and so no third sex—has appeared in multicellular animals."[80][81][82] Why sexual reproduction has an exclusively binary gamete system is not yet known. A few rare species that push the boundaries of the definitions are the subject of active research for light they may shed on the mechanisms of the evolution of sex. For example, the most toxic insect,[83] the harvester ant Pogonomyrmex, has two kinds of female and two kinds of male. One hypothesis is that the species is a hybrid, evolved from two closely related preceding species. Fossil records indicate that sexual reproduction has been occurring for at least one billion years.[84] However, the reason for the initial evolution of sex, and the reason it has survived to the present are still matters of debate, there are many plausible theories. It appears that the ability to reproduce sexually has evolved independently in various species on many occasions. There are cases where it has also been lost, notably among the Fungi Imperfecti.[85] The blacktip shark (Carcharhinus limbatus), flatworm (Dugesia tigrina) and some other species can reproduce either sexually or asexually depending on various conditions.[86] Sex/Gender taxonomy The following systematic list (gender taxonomy) illustrates the kinds of diversity that have been studied and reported in medical literature. It is placed in roughly chronological order of biological and social development in the human life cycle. The earlier stages are more purely biological and the latter are more dominantly social. Causation is known to operate from chromosome to gonads, and from gonads to hormones. It is also significant from brain structure to gender identity (see Money quote above). Brain structure and processing (biological) that may explain erotic preference (social), however, is an area of ongoing research. Terminology in some areas changes quite rapidly to accommodate the constantly growing knowledge base. chromosomes 46xx, 46xy, 47xxy (Klinefelter syndrome), 45xo (Turner's syndrome), 47xyy, 47xxx, 48xxyy, 46xx/xy mosaic, other mosaic, and others gonads testicles, ovaries, one of each (hermaphrodites), ovotestes, or other gonadal dysgenesis hormones androgens including testosterone; estrogens—including estradiol, estriol, estrone; antiandrogens and others genitals primary sexual characteristics (six class system) secondary sexual characteristics dimorphic physical characteristics, other than primary characteristics (most prominently breasts or their absence) brain structure special kinds of secondary characteristics, due to their influence on psychology and behaviour gender identity psychological identification with either of the two main sexes gender role social conformity with expectations for either of the two main sexes erotic preference gynophilia, androphilia, bisexuality, asexuality and various paraphilias. Sexual/gender dimorphism See also: Sexual differentiation, Sexual dimorphism, and Sex differences in humans Sexual differentiation in peafowl Although sexual reproduction is defined at the cellular level, key features of sexual reproduction operate within the structures of the gamete cells themselves. Notably, gametes carry very long molecules called DNA that the biological processes of reproduction can "read" like a book of instructions. In fact, there are typically many of these "books", called chromosomes. Human gametes usually have 23 chromosomes, 22 of which are common to both sexes. The final chromosomes in the two human gametes are called sex chromosomes because of their role in sex determination. Ova always have the same sex chromosome, labelled X. About half of spermatozoa also have this same X chromosome, the rest have a Y-chromosome. At fertilization the gametes fuse to form a cell, usually with 46 chromosomes, and either XX female or XY male, depending on whether the sperm carried an X or a Y chromosome. Some of the other possibilities are listed above. In humans, the "default" processes of reproduction result in an individual with female characteristics. An intact Y-chromosome contains what is needed to "reprogram" the processes sufficiently to produce male characteristics, leading to sexual differentiation. Part of the Y-chromosome, the Sex-determining Region Y (SRY), causes what would normally become ovaries to become testes. These, in turn, produce male hormones called androgens. However, several points in the processes have been identified where variations can result in people with atypical characteristics, including atypical sexual characteristics. Terminology for atypical sexual characteristics has not stabilized. Disorder of sexual development (DSD) is used by some in preference to intersex, which is used by others in preference to pseudohermaphroditism. Androgen insensitivity syndrome (AIS) is an example of a DSD that also illustrates that female development is the default for humans. Although having one X and one Y chromosome, some people are biologically insensitive to the androgens produced by their testes. As a result, they follow the normal human processes that results in a female. Women who are XY report identifying as a woman—feeling and thinking like a woman—and, where their biology is completely insensitive to masculinizing factors, externally they look identical to other women. Unlike other women, however, they cannot produce ova, because they do not have ovaries. The human XY system is not the only sex determination system. Birds typically have a reverse, ZW system—males are ZZ and females ZW.[citation needed] Whether male or female birds influence the sex of offspring is not known for all species. Several species of butterfly are known to have female parent sex determination.[87] The platypus has a complex hybrid system, the male has ten sex chromosomes, half X and half Y.[88] |
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