The relationship between biology and sexual orientation is a subject of research. While scientists do not know the exact cause of sexual orientationthey theorize that a combination of genetic, hormonal, and social factors determine it. Biological theories for explaining the causes of sexual orientation are favored by scientists  and involve Kin selection theory homosexuality statistics complex interplay of genetic factors, the early uterine environment and brain structure.
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, fantasy 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  found same-sex behavior was explained by both heritable factors Kin selection theory homosexuality statistics individual-specific environmental sources such as prenatal environment, experience with illness and trauma, as well as peer groups, and sexual experienceswhile influences of shared-environment variables such as familial environment and social attitudes had Kin selection theory homosexuality statistics weaker, but significant Kin selection theory homosexuality statistics. Women showed a statistically non-significant trend 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 Kin selection theory homosexuality statistics 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 moderate, primarily genetic, familial effects, and moderate to large effects of the nonshared environment social and biological on same-sex sexual behavior. Twin studies have 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 many sets of identical twins, sexual orientation cannot be attributed solely to genetic factors. Another issue is the recent finding that even monozygotic twins can be different and there is a mechanism which might account 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 amniocity.
Monoamniotic twins share a hormonal environment, but can suffer from the 'twin to twin transfusion syndrome' in which one twin is "relatively stuffed with blood and the other exsanguinated". linkage studies of sexual orientation have indicated the presence of multiple contributing
Kin selection theory homosexuality statistics factors throughout the genome. 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 X chromosome to test for similar alleles. This was popularly dubbed the " gay gene " in the media, causing significant controversy. A later analysis by Hu et al. In the full sample they did not find to Xq Results from the first large, comprehensive multi-center genetic linkage study of male sexual orientation were reported by an independent group of researchers at the American Society of Human Genetics in Significant linkage was also detected in the pericentromeric region of chromosome 8, overlapping with one of the regions detected in the
Kin selection theory homosexuality statistics lab's previous genomewide study.
The authors concluded that "our findings, taken in context with previous work, suggest that genetic variation in each of these regions contributes to development of the important psychological trait of male sexual orientation".
Female sexual orientation does not seem to be linked to Xq28,   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 type 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 common fruit fly Drosophila melanogasterthe complete pathway of sexual differentiation of the brain and the Kin selection theory homosexuality statistics it controls is well established in both males and females, providing a concise model of biologically controlled courtship.
Without the gene, the mice exhibited 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 genetic influences should not be equated with genetic determinism. According to Kin selection theory homosexuality statistics Hamer and Michael Bailey, genetic aspects
Kin selection theory homosexuality statistics only one of the multiple causes of homosexuality.
InNature 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 hypothalamus of gay men is different from straight men.
The researchers found another gene, named "thyroid stimulating hormone receptor" TSHR on chromosome 14 which dna sequence is different also for gay men. The previous research also indicated that grave disease had been seen more in gay men than in straight men.
It had been presumed that the overactive TSHR hormone lowered body weight in gay people. A study
Kin selection theory homosexuality statistics linkage between a mother's genetic make-up and homosexuality of her sons. Women have two X chromosomes, one 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 non-random fashion. This maternal immunization hypothesis MIH begins when cells from a male fetus enter the Kin selection theory homosexuality statistics circulation during pregnancy or while giving birth.
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.
Kin selection theory homosexuality statistics male fetuses are then attacked by H-Y antibodies which somehow decrease the ability of H-Y antigens to perform their usual function in brain masculinisation. 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 is responsible for a later son being gay. The result also indicates that number of pregnancies, mothers of gay sons, particularly those Kin selection theory homosexuality statistics 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 Kin selection theory homosexuality statistics the relatives of 98 homosexual and heterosexual men. Female relatives of the homosexual men tended to have more offspring than those of the heterosexual men.
Female relatives of the homosexual Kin selection theory homosexuality statistics on their mother's side tended to have more
Kin selection theory homosexuality statistics 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  has suggested that gay and straight men respond differently to two odors that are believed to be involved in sexual arousal. The research showed that when both heterosexual women and gay men are exposed to a testosterone derivative Kin selection theory homosexuality statistics in men's sweat, a in the hypothalamus is activated.
Heterosexual men, on the other 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 studying young subjects to see if similar Kin selection theory homosexuality statistics 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; Kin Kin selection theory homosexuality statistics theory homosexuality statistics is, they vary between men and women.
There have also been of variations brain structure corresponding to sexual 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 male or a female brain, and this mirrors the behavioral differences between the Kin selection theory homosexuality statistics sexes. Some researchers state that solid scientific support for this is lacking.
Although consistent differences Kin selection theory homosexuality statistics been identified, including the size of the brain and of specific brain regions, male and female brains are very similar. Simon LeVaytoo, conducted some of these early researches. This was a relevant area of the brain to study, because of evidence that it played a role in the regulation of sexual behaviour in animalsand because INAH2 and INAH3 had previously been reported to differ in size between men and women.
He obtained brains from 41 deceased hospital patients. The subjects were classified into three groups. The Kin selection theory homosexuality statistics group comprised 19 gay men who had died of AIDS -related illnesses. The second group 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 third 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 Kin selection theory homosexuality statistics patient groups were all identified from medical 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 basis of the numerical preponderance of heterosexual men in the population". However, the INAH3 group appeared to be twice as big in the heterosexual male group as in the gay male Kin selection theory homosexuality statistics the difference was highly significant, and remained significant 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 Kin selection theory homosexuality statistics in homosexual males who died of AIDS to heterosexual males, and therefore larger than 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 Kin selection theory homosexuality statistics twice as many as in heterosexual males. These areas of the hypothalamus have not yet been explored in homosexual females nor bisexual males nor females. William Byne and colleagues attempted to identify the size differences reported in INAH 1—4 by replicating the experiment using brain Kin selection theory homosexuality statistics from other subjects: The researchers found a significant difference in INAH3 size between heterosexual men and heterosexual women.
The size of the homosexual men was apparently smaller than that of the heterosexual men, and larger than that of the heterosexual women, though neither difference quite reached statistical significance.
The results for INAH3 weight were similar to those for INAH3 size; that is, the INAH3 weight for the heterosexual male brains was significantly larger than for the heterosexual female brains, while the results for the gay male group were between those of the other two groups but not quite significantly different from either.
The neuron count also found a male-female difference in INAH3, but found no trend related to sexual orientation. A study, Garcia-Falgueras and Kin selection theory homosexuality statistics asserted that "the Kin selection theory homosexuality statistics brain develops during the intrauterine period in the male direction through a direct action of testosterone on the developing nerve cells, or in the
Kin selection theory homosexuality statistics direction through the absence of this hormone surge.
In this way, our gender identity the conviction of belonging to the male or female gender and sexual orientation are programmed or organized into our brain structures when we are still in the womb. There is no indication that social environment after birth has an effect on gender identity or sexual orientation. In many species, a prominent feature of sexual differentiation is the presence of a sexually dimorphic nucleus SDN in the preoptic hypothalamus, which is larger in males than in females.
Neurons of the oSDN show aromatase expression which is also smaller in male-oriented rams versus female-oriented rams, suggesting that sexual orientation is neurologically hard-wired and may be influenced by hormones. However, results failed to associate the role of neural aromatase in the sexual differentiation of brain and behavior in the sheep, due to the lack of defeminization of adult Kin selection theory homosexuality statistics partner preference or oSDN volume as a result of aromatase activity in the brain of the fetuses during the critical period.
Having said this, it is more likely that oSDN morphology and homosexuality may be programmed through an Kin selection theory homosexuality statistics receptor that does not involve aromatisation.
Most of the data suggests that homosexual rams, Kin selection theory homosexuality statistics 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.
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