J. Kallman's and W. W. Schlegel's findings in the 1940s showed that twins had a 100 percent concordance rate for homosexual orientation, where concordance is defined as the level of similarity existing for different characteristics. Although the results seemed overwhelmingly high, they laid the base for further studies. In 1991, J. Michael Bailey and Richard C. Pillard conducted a similar experiment comparing identical twins, fraternal twins, and nongenetically related adopted brothers. The genetics of sexual orientation are not quite so straightforward, but there is no doubt that becoming gay is influenced by a genetic predisposition. In one study, it was reported that 52% of monozygotic twins of gay men were also gay, but this figure dropped to 22% for dizygotic twins. (1)
Dr. Hamer in 1991 from the Department of Biochemistry at the national Institute of Health began a study - possible genetic natire of homosexuality. He chose to study markers on the X chromosome because there was already evidence for a preponderance of gay men on the maternal side of families. Hamer hypothesized the different ways that a genetic disorder could be expressed in a person. He began theorizing around both autosomal dominant and autosomal recessive inheritances, but realized neither could work. For the gay gene to be autosomal dominant, 50% of the children of a family would be homosexual, and while this fact was true with some of his volunteers, there were not sufficient amounts of data to validate the hypothesis. An autosomal recessive inheritance would result in 25% of the children to be homosexual, another fact that was not easily supported. Hamer's breakthrough came during the interview of one of his volunteers; the volunteer suggested that the gene could be hidden on one of the sex chromosomes. Hamer continued his probing and research. After mapping out family trees, he collected DNA samples from his volunteers and their mothers, making extra samples and storing them for later analysis. (2)As a trial run, he began to test markers on the X chromosomes of the samples of blood from the gay volunteers. Using sib-pairs, a way of telling if brothers are concordant (sharing the same X chromosome from their mother) or discordant (one child inheriting one X chromosome from their mother, and the other child inheriting the second X chromosome), Hamer quickly learned that most of the gay brothers were concordant, therefore making it highly possible that a genetic link could be found. He began testing in random clusters, focusing around the region Xq28 (X for the chromosome, q for the arm, and 28 for the position on the arm).
Researchers say it's the first time the entire human genetic makeup has been scanned in search of possible genetic determinants of male sexual orientation. The results suggest that several genetic regions may influence homosexuality. Identical twins, for instance, share the same set of chromosomal patterns. Therefore, if one twin's DNA has a homosexual genetic trait, then it is inevitable that both twins will be homosexuals. However, that is not the case with all twins. When one twin is homosexual, the probability of the other identical twin being homosexual is 50 percent. Thus, the "gay"-gene theory