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A collaborative study by researchers from the University of East London and the Institute of Psychiatry of King's College London measured the startle response, or eye blink reaction, of heterosexuals and homosexuals who were subjected to loud noises.
Using a technique called prepulse inhibition, researchers unexpectedly startled subjects with a loud noise preceded by a quieter noise.
"The startle response is preconscious and cannot be learned," Qazi Rahman from the University of East London explains. "It is mediated by an ancient region of the brain called the limbic system which also controls sexual behavior."
Results showed clear differences between homosexual and heterosexual groups
Rahman, Q. "Sexual Orientation Related Differences in Prepulse Inhibition of the Human Startle Response" Behavioral Neuroscience magazine, 2003-OCT,
The fruitless gene in Drosophila produces male-specific protein (FRU(M)) involved in the control of courtship. FRU(M) spatial and temporal patterns were examined in fru mutants that exhibit aberrant male courtship. Chromosome breakpoints at the locus eliminated FRU(M). Homozygous viable mutants exhibited an intriguing array of defects. In fru(1) males, there were absences ofFRU(M)-expressing neuronal clusters or stained cells within certain clusters,reductions of signal intensities in others, and ectopic FRU(M) expression in novel cells. fru(2) males exhibited an overall decrement of FRU(M) expression inall neurons normally expressing the gene. fru(4) and fru(sat) mutants onlyproduced FRU(M) in small numbers of neurons at extremely low levels, and noFRU(M) signals were detected in fru(3) males. This array of abnormalities was inferred to correlate with the varying behavioral defects exhibited by these mutants. Such abnormalities include courtship among males, which has been hypothesized to involve anomalies of serotonin (5-HT) function in the brain.However, double-labeling uncovered no coexpression of FRU(M) and 5-HT in brainneurons. Yet, a newly identified set of sexually dimorphic FRU(M)/5-HT-positive neurons was identified in the abdominal ganglion of adult males. These sexually dimorphic neurons (s-Abg) project toward regions of the abdomen involved in male reproduction. The s-Abg neurons and the proximal extents of their axons were unstained or absent in wild-type females and exhibited subnormal or no 5-HTimmunoreactivity in certain fru-mutant males, indicating that fruitless controls the formation of these cells or 5-HT production in them."
Drosophila.Lee G, Hall Abnormalities of male-specific FRU protein and serotonin expression in the CNS of fruitless mutants in JC. Journal of Neuroscience 2001 Jan 15;21(2):513-26
"We recruited twins systematically from the Australian Twin Registry and assessedtheir sexual orientation and 2 related traits: childhood gender nonconformityand continuous gender identity. Men and women differed in their distributions of sexual orientation, with women more likely to have slight-to-moderate degrees of homosexual attraction, and men more likely to have high degrees of homosexual attraction. Twin concordances for nonheterosexual orientation were lower than in prior studies. Univariate analyses showed that familial factors were important for all traits, but were less successful in distinguishing genetic from shared environmental influences. Only childhood gender nonconformity was significantly heritable for both men and women. Multivariate analyses suggested that the causal architecture differed between men and women, and, for women, provided significant evidence for the importance of genetic factors to the traits' covariation."
Bailey JM, Dunne MP, Martin NG. J Pers, Genetic and environmental influences on sexual orientation and its correlates in an Australian twin sample. Social Psycholgy 2000 Mar;78(3):524-36
"Research has generally supported the existence of familial-genetic factors for male sexual orientation, but has not shed much light on the specific nature of those influences. Gay men with gay brothers provide the opportunity to examine several hypotheses. Sixty-six men, representing 37 gay male sibling pairs,completed questionnaires assessing behavior on various measures includingchildhood and adult gender nonconformity, timing of awareness of homosexualfeelings, self-acceptance, and the quality of family relationships. Consistentwith prior findings using twins, gay brothers were similar in their degree ofchildhood gender non-conformity, suggesting that this variable may distinguish etiologically (e.g., genetically) heterogeneous subtypes. The large majority of gay men with brothers knew about their own homosexual feelings before they learned about their brothers' homosexual feelings, suggesting that discovery of brothers' homosexuality is not an important cause of male homosexuality."
Dawood K, Pillard RC, Horvath C, Revelle W, Bailey JM. Familial aspects of male homosexuality. Archives of Sexual Behavior 2000 Apr;29(2):155-63
"The survival of a human predisposition for homosexuality can be explained by sexual orientation being a polygenetic trait that is influenced by a number of genes. During development these shift male brain development in the female direction. Inheritance of several such alleles produces homosexuality. Single alleles make for greater sensitivity, empathy, tender mindedness, and kindness.These traits make heterosexual carriers of the genes better fathers and more attractive mates. There is a balanced polymorphism in which the feminizing effect of these alleles in heterosexuals offsets the adverse effects (onreproductive success) of these alleles' contribution to homosexuality. A similar effect probably occurs for genes that can produce lesbianism in females. The whole system survives because it serves to provide a high degree of variabilityamong the personalities of offspring, providing the genotype with diversification and reducing competition among offspring for the same niches. An allele with a large effect can survive in these circumstances in males, but it is less likely to survive in females. The birth order effect on homosexuality is probably a by-product of a biological mechanism that shifts personalities more in the feminine direction in the later born sons, reducing the probability of these sons engaging in unproductive competition with each other."
Miller EM. Homosexuality, birth order, and evolution: toward an equilibrium reproductive economics of homosexuality. Archives of Sexual Behavior 2000 Feb;29(1):1-34
"The study of Drosophila melanogaster by a combination of forward genetics with specific mutants, and reverse genetics, in which a given gene is expressed in an appropriate brain area to test its effect on behavior, provides a unique opportunity to explore the causal relationship between a particular gene, its function in the cell and the behavioral outcome at the organismic level. Enhanced male-to-male courtship has been shown to occur as a result of mutations in several different genes. For example, the Voila mutant exhibits intense GAL4 reporter expression in the tarsal gustatory sensilla, suggesting the importance of tapping by a male on the female abdomen with his forelegs. Feminization of parts of the antennal lobe and mushroom body by targeted expression of afemale-determining gene transformer+ (tra+) drives the male to court other males. Mutations in the tra target gene fruitless (fru), which is expressed inthe antennal lobe as well as the suboesophageal ganglion (the gustatory inputsare processed here), also induce homosexual courtship in males. These resultssuggest that sensory inputs mediated and/or processed by the tarsal receptors,suboesophageal ganglion, antennal lobe and mushroom body contribute to theregulation of male-female courtship. Mosaic analysis localized the neural centerfor male courtship behavior to the posterior dorsal brain, in which the sensoryinformation processed by the aforementioned neural structures may be integrated.Another mosaic study mapped the neural center for female sexual behavior, asmeasured by her receptiveness to copulation, to the anterior dorsal brain. Theissue as to how the mutations that reduce female sexual receptiveness, e.g.dissatisfaction (dsf), spinster (spin) and chaste (cht), affect the structureand/or function of this neural center deserves to be addressed urgently."
Yamamoto D, Nakano Y Cell Mol Sexual behavior mutants revisited: molecular and cellular basis of Drosophila mating. Life Science 1999 Nov 15;56(7-8):634-46
"The white gene encodes an ABC-type transmembrane transporter that has a role in normal eye pigment deposition. In addition, overexpression in Drosophila leads to homosexual male courtship. Its human homologue has been implicated in cholesterol transport in macrophages and in mood disorders in human males. The garnet gene is a member of a group of other Drosophila eye colour genes that have been shown, or proposed, to function in intracellular protein transport. Recent molecular analysis indicates that it encodes the delta subunit of the AP-3 adaptin complex involved in vesicle transport from the trans-Golgi network to lysosomes and related organelles, such as pigment granules. This identification revealed a novel role for intracellular vesicular transport in Drosophila pigmentation. To further analyze this intracellular transport system, we examined the genetic interactions between garnet and a second site enhancer mutation, enhancer of garnet (e(g)). We show here that e(g) is a cryptic allele of the white gene. The white-garnet interaction is highly sensitive to the levels of both gene products but also shows some allele specificity for the white gene. The additive effect on pigmentation and the predicted protein products of these genes suggest that the garnet/AP-3 transport system ensures the correct intracellular localization of the white gene product. This model is further supported by the observation of homosexual male courtship behavior in garnet mutants, similar to that seen in flies overexpressing, and presumably mis-sorting, the white gene product. The w(e(g)) allele also enhances mutationsin the subset of other eye-color genes with phenotypes similar to garnet. This observation supports a role for these genes in intracellular transport and leads to a model whereby incorrect sorting of the white gene product can explain the pigmentation phenotypes of an entire group of eye-color genes."
Lloyd VK, Sinclair DA, Alperyn M, Grigliatti TA. Enhancer of garnet/deltaAP-3 is a cryptic allele of the white gene andidentifies the intracellular transport system for the white protein. Genome. 2002 Apr;45(2):296-312.
There exists several hundred scientific studies published in peer reviewed journals showing a biological origin for sexual orientation. They are not poorly constructed as you would like to pretend