In all honesty, I've never been convinced of the biological determination of sexual orientation. The few studies that "seem to" confirm this are not empirically substantive. The failure of identical twin studies to prove a genetic imperative is a serious deterrent to my signing on to such a belief. However, if there are empirically-proven intrauterine as well as genetic precursors to sexual-orientation-associated temperament, for instance (bearing in mind how difficult temperament is to measure, let alone prove), then I am personally more likely to subscribe to the notion that one's sexual predilections and tendencies can be influenced by prenatal factors. Intrauterine hormone exposure may provide a link to such a sensibility; this is the angle – if one exists - most likely to sway my own opinion at this point, though still not toward a so-called "gay gene". What are some biopsychiatrists saying, and how does this relate to Dr. Peter D'Adamo's determinations of Genotype?

Prenatal exposure to one or the other pole of sex hormones has been being linked by some researchers in biopsychiatry to sexual orientation/preference/identification for well over a decade now, as but one of several factors of influence. A couple of days ago I saw a paragraph in an online textbook stating that:
- homosexual men may exhibit lower levels of circulatory androgens than heterosexuals do, and that
- the effective presence of androgens in prenatal life contributes toward a sexual predilection toward females for all, while
- a "deficiency of androgens - or tissue insensitivity to these" may lead one to be oriented toward males. (Kaplan & Sadock's Synopsis of Psychiatry).
I've also seen a study linking the opposite factor - HIGH prenatal androgen exposure - to male homosexuality. (You've been warned: Not only is acceptance of the "science" NOT universal; even the "convinced" scientists can disagree with one another.)

This caught my eye because Dr. Peter D'Adamo, in his The Genotype Diet, treats of D2: D4 (the prenatal sex-hormonally influenced ratio of index finger length to that of the ring finger) as a determinant of Genotype within his 6-type system. He writes that this ratio is "an excellent marker for sex hormone exposure. A longer ring finger means you encountered more androgens in the womb (androgens are a testosterone precursor); a long index finger means you faced higher levels of estrogen."

According to numerous respected biopsychiatry sources, D2: D4 does somewhat correlate with sexual orientation. Longer D2 (index finger) is a marker of prenatal dominance of estrogen exposure; longer D4 (ring finger) of that of androgens. Females with high androgen exposure, and males with high estrogen exposure, are apparently likelier than most to experience same-sex attractions. (I haven't yet seen a good explanation as to WHY/HOW: The "route".)

According to Dr. D'Adamo, D2: D4 definitely correlates with Genotype; remembering this, my sudden enquiry was provoked.

Here are some summarised findings of D'Adamo with respect to D2: D4 prevalence within the six genotype populations, per The Genotype Diet (2007):

GT-1 Hunters tend toward longer D4 on both hands
GT-2 Gatherers tend toward longer D2 on both hands. Also often seen is hand-asymmetry for which finger's length is greater (i.e., a difference between the two hands), a factor other researchers correlate with deviations from heterosexuality.
GT-3 Teachers may tend toward longer D2 on both hands. Or, like GT2, asymmetry between hands.
GT4 -Explorers: Longer D4 on both hands, especially among females. From another page, some likelihood that men are more prone to longer D2, women to longer D4.
GT5- Warriors tend toward symmetry between hands.
GT6: Nomads tend toward longer D2 on both hands for females, longer D4 on both hands for men.

IF D2: D4 were always and only a result of prenatal androgenic/estrogenic dominance, and IF this predominance were a surefire determinant of sexual orientation, we'd be able to predict someone's sexual preference by measuring his/her D2: D4! In that case, and only in that case, we'd expect to see – per Dr. D'Adamo's correlations of D2: D4 ratios with Genotype – homosexuality somewhat more common among:

Hunter women
Gatherer men
Teacher men
Explorer women and men

(Warriors wouldn't correlate in any direction.
The Nomad genotype would skew strongly toward heterosexuality.)

This might automatically imply (if all bloodtypes and genotypes have no gender-preponderance) that the O bloodtype population (which contains only Hunters, Gatherers, and Explorers) is more likely to be associated with homosexuality than are the other bloodtypes.

IF you subscribe to the Genotype system, AND to the prenatal androgen/estrogen theory regarding sexual orientation, a Genotype/sexual-orientation link has interesting implications, including that certain diets and fitness-practices could be statistically associated with greater or lesser health among homosexuals. If, in the end, homosexuality were to definitively prove to be related to Genotype, sexual preference would be an important datum to consider in the "strength-testing" of genotype for type-determination and in diet/nutrient/fitness prescription according to D'Adamo's increasingly popular genotyping software, SWAMI. This is where my interest was piqued. Conclusive data here would be revolutionary, to say the least.

Feel free to supply any related bloodtype/genotype-related information you may have come across on this compelling subject.