I expect there to be more information of the genetetic aspects of this in the near future. As it is there has already been hormones identified that influences gender identity. While obviously we enter our development in utero having chromosomes that determine our gender, there are certain hormones released in the process and timing is very important. These hormones control genital development and ultimately influence gender identity. Basically we all have nothing, or start out female. When certain hormones are released males develop penises and the other differences that make them distinctively male like higher testosterone production. There is a lot of variance, and sometimes these hormones are released late, are absent, etc. The result can be fetuses that have penises, but lack the other hormones that link to being a male and instead have high estrogen instead of testosterone. Some girls produce more testosterone than estrogen, and some of these females have enlarge clitorises that actually resemble penises. With all this variance it is clear that even biological sexuality is not black and white, so it is no surprise that gender identity is not black and white either. The result is people who even as infants/small children have an gender identity contrary to their physical body. In essence, they are born gay. There has been a lot of research supporting this and obviously I summated it in a simplified manner, but you cannot discount that biology has a great influence on this as well as environment.
The main thing to remember is that there hasn't been any conclusive evidence to support a genetic predetermination to homosexuality. Genetic is directly inherited: color of eyes, hair, etc. We all know this. There are, however, things that point to homosexuality being influenced by other factors. The below list is take from Dr. Jeffrey Satinover's book "Homosexuality and the Politics of Truth."
1. Genetic- as mentioned above [ Dr. Satinover lists what we genetic traits to be, i.e., recessive, dominant, etc]
2. Not genetic, but caused by intrauterine influences. These are traits (such as the degree to which a fetus develops masculine or feminine sexual characteristics) that are influenced by various aspects of the environment in the womb. Hormones, infections, exercise, general health, the ingestion of licit or illicit drugs, and many out variables influence this environment. Thus one may be born with a trait that is innate, but not genetic.
Familial-- meaning that they tend to be shared by members of the same family. Familial traits may be:
1 Genetic. Because they have the same parents, brothers and sisters are more likely to share a high percentage of similar genes that would unrelated individuals.
2. Innate, but not genetic. Sharing the same mother, certain typical factors may remain constant or similar for all children born to her. Examples include the effect of her dietary habits on her unborn children, the fact that she smoked, or her general health.
3. Not innate, but environmental. To an extent greater than between individuals from different families, individuals raised in the same family share a similar environment. These include the physical, emotional, and moral influences. Thus family members may share some traits that are neither genetic nor innate but that are nonetheless transmitted from one generation to the next by influence.
A biological trait is rooted in an organism's physiology, rather than its psychology. With respect to behavioral traits, this distinction suggest a dichotomy comparable to the difference between "hardware" and "software" in the domain of computer science.
2. Innate but not genetic
3. Environmental and familial but not innate (for example, the effect of a virus that has taken root among the members of a household).
4. Environmental and not familial and possibly innate but maybe not (for example, the effect of a toxin in the environment at large, depending on whether its baleful influence is felt pre- or postnatally).
Additionally, the cause of a trait may be purely environmental but not biological-- at least insofar as we do not attend to the biological dimension. Examples include the influence on behavior of the values, standards, habits, economic status, and so on, of a family or society.
Direct versus Indirect
Finally, any of these causes may be direct or indirect. That is,
1. Lead directly to the trait. Whether we are speaking of genetic or nongenetic, innate or noninnate, biological or nonbiological influences, the cause may directly produce the trait itself, as when genes cause blue eyes or when smoke causes a cough.
2. Lead indirectly to the trait. Because of what the influence causes directly, the individual finds it desirable to choose a particular trait. This is seen, for example, when tall athletic individuals become basketball players or when short athletic people become jockeys.
Furthermore, all of these causes may combine and influence one another in highly interdependent ways, mutually influencing each other throughout a lifetime. Behavioral traits, as opposed to simple, single-gene physiologic traits such as eye color, always interact in this way.
In summary, the question concerning all behavioral traits, such as homosexuality, cannot be "Is such and such genetic?" Rather we must ask, "To what extent, respectively, is such and such genetic and nongenetic, innate and noninnate, familial and nonfamilial, environmentally determined and not, direct and indirect? In the course of development, when do which influences dominate and how do their interactions affect one another?" We need to keep this sobering caution in mind as we clarify what medical science has and has not learned about the subject of homosexuality.