Those mice who retained the gene fucose mutarotase FucM were attracted to male mice. Furthermore, the SCN of homosexual males is extremely large both the volume and the number of neurons are twice as many as in heterosexual males. Six of these men had died of AIDS-related illnesses. Some researchers state that solid scientific support for this is lacking. Lesbian women and straight men have, on average, slightly larger right brain hemispheres.
The inactivation of the X chromosome occurs randomly throughout the embryo, resulting in cells that are mosaic with respect to which chromosome is active. Without the gene, the mice exhibited masculine sexual behavior and attraction toward urine of other female mice. Heterosexual men, on the other hand, have a similar response to an estrogen-like compound found in women's urine. More research is needed in order to understand the requirements and timing of the development of the oSDN and how prenatal programming effects the expression of mate choice in adulthood. Brendan Zietsch of the Queensland Institute of Medical Research proposes the alternative theory that men exhibiting female traits become more attractive to females and are thus more likely to mate, provided the genes involved do not drive them to complete rejection of heterosexuality. Female relatives of the homosexual men on their mother's side tended to have more offspring than those on the father's side. Several theories have been advanced to explain this contradiction, and new experimental evidence has demonstrated their feasibility. This was popularly dubbed the "gay gene" in the media, causing significant controversy. Vasey and VanderLaan tested the theory on the Pacific island of Samoa, where they studied women, straight men, and the fa'afafine, men who prefer other men as sexual partners and are accepted within the culture as a distinct third gender category. Also, the authors of the study acknowledge that a large number of sexual partners may not lead to greater reproductive success, specifically noting there is an "absence of evidence relating the number of sexual partners and actual reproductive success, either in the present or in our evolutionary past". Most of the data suggests that homosexual rams, like female-oriented rams, are masculinized and defeminized with respect to mounting, receptivity, and gonadotrophin secretion, but are not defeminized for sexual partner preferences, also suggesting that such behaviors may be programmed differently. Hofman reported a difference in the size of the suprachiasmatic nucleus between homosexual and heterosexual men. The research showed that when both heterosexual women and gay men are exposed to a testosterone derivative found in men's sweat, a region in the hypothalamus is activated. The third group was of six women whom the researchers presumed to be heterosexual. Since the hypothesis solves the problem of why homosexuality has not been selected out over thousands of years, despite it being antithetical to reproduction, many scientists consider it the best explanatory model for non-heterosexual behaviour such as homosexuality and bisexuality. The records of the remaining heterosexual subjects contained no information about their sexual orientation; they were assumed to have been primarily or exclusively heterosexual "on the basis of the numerical preponderance of heterosexual men in the population". However, other studies have shown that the sexually dimorphic nucleus of the preoptic area, which include the INAH3, are of similar size in homosexual males who died of AIDS to heterosexual males, and therefore larger than female. Furthermore, the SCN of homosexual males is extremely large both the volume and the number of neurons are twice as many as in heterosexual males. These Y-linked proteins would not be recognized in the mother's immune system because she is female, causing her to develop antibodies which would travel through the placental barrier into the fetal compartment. LGBT social movements and LGBT rights opposition Whether genetic or other physiological determinants form the basis of sexual orientation is a highly politicized issue. Some researchers state that solid scientific support for this is lacking. Prenatal hormones and sexual orientation The early fixation hypothesis includes research into prenatal development and the environmental factors that control masculinization of the brain. Haldane in and later elaborated by many others including John Maynard Smith , W. In some cases though, it appears that this switching off can occur in a non-random fashion. They conjure up the specter of the surgical or chemical "rewiring" of gay people, or of abortions of fetal homosexuals who have been hunted down in the womb.
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