Hamilton and Mary Jane West-Eberhard. 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. 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. Vasal and VanderLaan provides evidence that if an adaptively designed avuncular male androphilic phenotype exists and its development is contingent on a particular social environment, then a collectivistic cultural context is insufficient, in and of itself, for the expression of such a phenotype. 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 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. According to Bem, this feeling of difference will evoke psychological arousal when the child is near members of the gender which it considers as being 'different'.
Female sexual orientation does not seem to be linked to Xq28,   though it does appear moderately heritable. Gay men and straight women have, on average, equally proportioned brain hemispheres. 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. In the common fruit fly Drosophila melanogaster , the complete pathway of sexual differentiation of the brain and the behaviors it controls is well established in both males and females, providing a concise model of biologically controlled courtship. This may correlate with left-handedness. Significant linkage was also detected in the pericentromeric region of chromosome 8, overlapping with one of the regions detected in the Hamer lab's previous genomewide study. The natural bell curve variation that occurs in biology and sociology everywhere, explains the variable spectrum of expression. There have also been reports of variations in brain structure corresponding to sexual orientation. Six of these men had died of AIDS-related illnesses. One of the women had died of an AIDS-related illness. Female relatives of the homosexual men tended to have more offspring than those of the heterosexual men. They think Neuroligin 4 Y-linked protein is responsible for a later son being gay. Haldane in and later elaborated by many others including John Maynard Smith , W. They also found that "unusually high" proportions of homosexual males and homosexual females were Rh negative in comparison to heterosexuals. The third group was of six women whom the researchers presumed to be heterosexual. 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. 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 previous research also indicated that grave disease had been seen more in gay men than in straight men. 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. The researchers found another gene, named "thyroid stimulating hormone receptor" TSHR on chromosome 14 which dna sequence is different also for gay men. 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 Advocate , a U. 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. The first group comprised 19 gay men who had died of AIDS -related illnesses. They argue that sexual orientation can shift over the course of a person's life.
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