If homosexualtiy is genetic then, since monozygotic twins are genetically identical, when one twin is gay then virtually 100% of the others should also be gay.
FACT: Only between 38% and 52% are gay. Proving now and forever that homosexuality is
NOT genetic!
Bailey and Pillard (1991): occurrence of homosexuality among brothers
52% of identical (monozygotic) twins of homosexual men were likewise homosexual
22% of fraternal (dizygotic) twins were likewise homosexual
11% of adoptive brothers of homosexual men were likewise homosexual
J.M. Bailey and R.C. Pillard, A genetic study of male sexual orientation, Archives of General Psychiatry, vol. 48:1089-1096, December 1991
Better research, however, was based on twins who were recruited for other reasons, and only subsequently asked about their sexual orientation. These are known as "registry" studies, and they similarly gave a concordance rate between identical twins of less than 50%. There have been two major published registry studies (4,5), one based on the Minnesota Registry, the other on the Australian Registry. The larger of the two registry studies is the Australian one, done by Bailey, Martin and others at the University of Queensland. Using the 14,000+ Australian twin collection, they found that if one twin was homosexual, 38% of the time his identical brother was too. For lesbianism the concordance was 30%. Whether 30% or 50% concordance (snowball samples), all the studies agree it is clearly not 100%.
The critical factor is that if one identical twin is homosexual, only sometimes is the co-twin homosexual. There is no argument about this in the scientific community.
The distribution of sexual orientation among cotwins of the monozygotic probands appeared to be bimodal. In other words, most subjects classified themselves as heterosexual or homosexual, with very few giving evidence of significant bisexuality. This finding is in agreement with other recent studies (e.g. Buhrich et al. 1991).
According to their data, 52% (29/56) of monozygotic cotwins, 22% (12/54) of dizygotic cotwins, and 11% (6/57) of adoptive brothers were homosexual. Heritabilities of homosexuality were calculated using these results under a wide range of assumptions of the population base rate and ascertainment bias. Under all conditions considered, heritabilities were substantial (h2 was between .31 and .74 in all cases). However, "the rate of homosexuality among nontwin biological siblings, as reported by probands, 9.2% (13/142), was significantly lower than would be predicted by a simple genetic hypothesis and other published reports."
Bailey-Pillard
In December of 1991, Michael Bailey of Northwestern University joined Richard Pillard of the Boston University School of Medicine in publishing a study of homosexuality in twins. Their conclusion is that sexual orientation is something one is born with.
Bailey and Pillard surveyed homosexual men about their brothers, and they found some statistics that were rather unexpected. Of the homosexuals who had identical twin brothers, 52 percent of those twins were also homosexual. 22 percent of those who had fraternal twins said that their twin was gay, and only eleven percent of those who had adopted siblings said that their adopted brothers were also homosexual.
Bailey and Pillard attributed the differences in these percentages to the difference in the amount of genetic material shared. Since identical twins have the same genetic code, they are far more likely to share sexual orientation than fraternal twins. In the same way, it is obvious that fraternal twins have more in common genetically than do their adopted siblings.