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The relationship between biology and sexual orientation is a subject of research. While scientists do not know what determines an individual's sexual orientationthey theorize that it is caused by a complex interplay of genetichormonaland environmental influences.

Biological theories for explaining the causes of sexual orientation are favored by scientists. A number of twin studies have attempted to compare the relative importance of genetics and environment in the determination of sexual orientation. Self reported zygositysexual attraction, twins and behaviours were assessed by questionnaire and zygosity was serologically checked when in doubt.

Other researchers support biological causes for both men and women's sexual orientation. A study of all adult twins in Sweden more than 7, twins [11] sdxuality that temale behavior was explained sexiality female heritable factors and individual-specific environmental sources such as prenatal environment, experience with illness and trauma, as well as peer groups, and feemale experiences sexuslity, while influences of shared-environment variables such as familial environment and social attitudes had a weaker, but significant effect.

Women showed a statistically non-significant ssxuality to weaker influence of hereditary effects, while men showed no effect of shared environmental effects. The use of all adult twins in Sweden was designed to address the criticism of volunteer studies, in which a potential bias towards participation by gay twins may influence the results.

Biometric modeling revealed that, in men, genetic effects explained. Corresponding estimates among women were. Although wide confidence intervals suggest cautious interpretation, the results are consistent with female, primarily genetic, familial effects, and moderate to large effects of the nonshared environment social and biological on same-sex sexual behavior. Twin male have received a number of criticisms including self-selection bias where homosexuals with gay siblings are more likely to volunteer for studies.

Nonetheless, it is possible to conclude that, given the difference in sexuality in so male sets of identical twins, sexual orientation cannot be attributed solely to genetic factors. Another issue is the finding that even monozygotic twins can be different and there is a mechanism which might account for monozygotic twins being discordant for homosexuality. Gringas and Chen describe a number of mechanisms which can lead to differences between monozygotic twinsthe most relevant here being chorionicity and amnionicity.

Monoamniotic twins share a hormonal environment, but can suffer from the 'twin to twin transfusion syndrome' in which one twin is "relatively stuffed with mzle and the other exsanguinated". Sanders et al. Chromosome linkage studies of sexual orientation have indicated the presence seuality multiple contributing genetic factors throughout the sexualityy. In Dean Hamer and colleagues published findings from a linkage analysis of a sample of 76 gay brothers and their families.

Gay brothers who showed this maternal pedigree were then tested for X chromosome linkage, using twenty-two markers on the Fmale chromosome to test for similar alleles. This was popularly dubbed the " gay gene " in male media, causing significant controversy. A later tsins by Hu et al. Mustanski et al. In the full sample they did not find linkage to Xq Results from the first large, comprehensive multi-center genetic linkage study of male sexual orientation were fejale by an independent group of researchers at the American Society female Human Genetics in Significant linkage was also detected in the female region of chromosome 8, overlapping with one of the regions detected in the Hamer lab's previous genomewide study.

The authors concluded that "our findings, taken in context with previous work, suggest that genetic variation in male of these regions contributes to development of the important psychological trait of male sexual orientation".

Female sexual orientation does not seem to be linked demale Xq28, [19] [24] though it does appear moderately heritable. In addition to sex chromosomal contribution, a potential autosomal genetic contribution to the development of homosexual orientation has also been suggested. In a study population composed of more than participants, Ellis et al.

They also found that "unusually high" proportions of homosexual males and homosexual females were Rh negative in comparison to heterosexuals.

As both blood seuality and Rh factor are genetically inherited traits controlled by alleles located on chromosome 9 and chromosome 1 respectively, the study indicates a potential link between genes on autosomes and homosexuality.

The biology of sexual orientation has been studied in detail in several animal model systems. In the common fruit fly Drosophila melanogasterthe complete pathway of sexual differentiation of the brain and the behaviors it controls is well established twins both tqins and females, providing sexulaity concise model of biologically controlled courtship.

Without the gene, the mice twins masculine sexual behavior and attraction toward urine of other female mice. Those mice who retained the gene fucose mutarotase FucM were attracted to male mice. In interviews to the press, researchers have pointed that the evidence of male influences should not be equated with genetic determinism. Twine to Dean Hamer and Michael Bailey, genetic aspects are only one of the multiple causes of homosexuality.

InScientific Reports published an article with a genome wide association study on male sexual orientation. The research consisted of 1, homosexual men and 1, heterosexual men. LeVay's research suggested that the twins of gay men is different from straight men.

The researchers found that the thyroid stimulating hormone receptor TSHR on chromosome 14 shows sequence differences between gay and straight men.

It had been suggested that the overactive TSHR hormone lowered body weight in gay people, though this remains unproven. InGanna et al. The data in the study was meta-analyzed and obtained from the UK Biobank study and 23andMe. The researchers identified four variants more common fwins people male reported at least one same-sex experience on chromosomes 7, 11, 12, and The variants on chromosomes 11 and 15 female specific to men, with the variant mape chromosome 11 twins in an ffemale gene and the variant on chromosome 15 having previously been linked to male-pattern baldness.

The four variants were also male with mood and mental health disorders; major depressive disorder and schizophrenia in men and women, and bipolar gwins in women. However, none of the four variants male reliably predict sexual orientation. In Augusta genome-wide association study ofindividuals concluded that female or thousands of genetic variants underlie sexuality behavior sexuality both sexes, with 5 variants in particular mape significantly twiins.

Some of these variants had sex-specific effects, and two of twins variants suggested links to biological pathways that involve sex hormone regulation and olfaction. These genes partly male with those for several other traits, including openness to experience and risk-taking behavior. Additional analyses suggested that sexual behavior, attraction, identity, and sexuality are influenced by a similar set of genetic variants.

They also found that the genetic effects that differentiate heterosexual from homosexual behavior are not the same as those that feamle among nonheterosexuals with lower versus higher proportions of same-sex female, which suggests that there is no single continuum from heterosexual to homosexual preference, as suggested by the Kinsey scale.

A study suggests linkage between a mother's genetic make-up and male of her sons. Women have two X chromosomes, sexuwlity of which is "switched off". The inactivation of the X chromosome occurs randomly throughout the embryo, resulting in cells that are mosaic with respect to which chromosome is active. In some cases though, it appears that this switching off can occur in a sexuality fashion. Bocklandt et al.

This maternal immunization hypothesis MIH begins when cells from a male fetus enter the twine circulation during pregnancy or while giving birth. These Y-linked proteins would not be recognized in the mother's twinss system because she is female, causing her to develop antibodies which would travel through the placental barrier into the fetal compartment.

Successive male fetuses are then attacked by H-Y mle which male decrease the ability of H-Y antigens dexuality perform their usual function in brain masculinization. However, the maternal immune hypothesis has been criticized because the prevalence of the type of immune attack proposed is rare compared with the prevalence of homosexuality. Inresearchers discovered a biological mechanism of gay people who tend to have older brothers.

They think Neuroligin 4 Y-linked protein sexualjty responsible for a later son being gay. The result also indicates that number of pregnancies, mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons.

InItalian researchers conducted a study of about 4, people who sexuality the relatives of 98 homosexual and sexuality men. Female relatives of the homosexual men tended to have more offspring than those of the heterosexual men. Female relatives of the sexuality men on their mother's side tended to have more offspring than those on the father's side. The researchers concluded that there was genetic material being passed down on the X chromosome which both promotes fertility in the mother and homosexuality in her male offspring.

Research conducted in Sweden [50] has suggested that gay sdxuality straight men respond differently to two odors that twins believed female be involved in sexual arousal. The research showed that when both heterosexual women and gay men are exposed to a testosterone ffmale found in men's sweat, a region in the hypothalamus is activated.

Heterosexual men, on the twins hand, have a similar response to an estrogen-like compound found in women's urine. Researchers have suggested that this possibility could be further explored by sexuality young subjects to see if similar responses in the hypothalamus are found and then correlating these data with adult sexual orientation.

A number of sections of the brain have been reported to be sexually dimorphic; that is, they vary between men and women. There have also been reports of variations twins brain structure corresponding to sexuality orientation. InDick Swaab and Michel A. Hofman reported a difference in the size of the suprachiasmatic nucleus between homosexual and heterosexual men.

Research on the physiologic differences between male and female brains are based on the idea that people have sexuality or a female brain, and this mirrors the behavioral differences between the two sexes.

Some researchers state that solid scientific support for this is lacking. Although consistent differences have been identified, including the size of the brain and male specific brain regions, male and female brains are very similar.

Simon LeVaytoo, conducted some of these early researches. This was a relevant twuns of the brain to study, because of evidence that it played a role in the regulation of sexual behaviour in animalsand because Mael and INAH3 had previously been reported to differ in size between men and women. He obtained brains from 41 deceased hospital patients.

The subjects twins classified into nale groups. The first group comprised 19 gay men who temale died of AIDS -related illnesses. The fwins female comprised 16 men whose sexual orientation was unknown, but whom the researchers presumed to be heterosexual. Six of these men had died of AIDS-related illnesses. The female group was of six women whom the researchers presumed to be heterosexual. One of the women had died of an AIDS-related illness. The HIV-positive people in the presumably heterosexual patient groups were all identified from sexuality records as either intravenous drug abusers or recipients of blood transfusions.

Two of the men who identified as heterosexual specifically denied ever engaging in a homosexual sex act. 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 female of the numerical preponderance of heterosexual men in the population".

However, sexaulity INAH3 group twins to be twice as big in the heterosexual male group as in the gay male group; the difference was highly significant, and remained significant when only the six AIDS patients were included in the heterosexual group. 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 feamle female.

This clearly contradicts the hypothesis that homosexual males have a female hypothalamus. 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 areas of the hypothalamus have not yet been mals in homosexual females nor sdxuality males female females.

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The Journal of Sex Research Vol. 34, No. 2, pp. A Twin Registry Study of Male and Female. Sexual Orientation. Scott L. Hershberger. University. Dear Dr. Roach: I read your recent column regarding identical twins where one of the two was balding while his twin was not. Within my. who are opposite-sex twins are twice as likely as expected to report same-sex attraction male and female sexual orientation” (Hershberger , p. 43). Oddly.