The basics on orientation: a natural human variation.  On this page:

For more detailed information on genetics, see the Genetics Primer.

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Over at the Advocate, geneticst Dean Hamer takes on the "is it natural" question. The whole thing is worth reading.
[T]he scientific community has long regarded sexual orientation – whether gay, straight, or somewhere in between – as a phenotype: an observable set of properties that varies among individuals and is deeply rooted in biology. For us, the role of genetics in sexual behavior is about as “disputable” as the role of evolution in biology. Come to think of it, pretty much the same folks are opposed to both ideas.

The empirical evidence for the role of genetics in sexual orientation has steadily mounted since I first entered the field in the early 1990s. Back then, the only quantitative data was derived from studies of unrepresentative and potentially biased samples of self-identified gay men and lesbian. But in the intervening 20 years, studies of twins – the mainstay of human population genetics – have been conducted on systematically ascertained populations in three different countries. These studies are notable because they have large sample sizes that are representative of the overall population, they’re conducted by independent university-based investigators using well-established statistical methods, and the results are published in the peer-reviewed literature.

Each of these studies has led to the same fundamental conclusion: genes play a major role in human sexual orientation. By contrast, shared environmental factors such as education, parenting style, or presumably even exposure to Lady Gaga, have little if anything to do with people's orientation. While there is a substantial amount of variation that cannot be ascribed to either heritable or shared environment, the differences might also be due to biological traits that are not inherited in a simple additive manner.....

Another criticism frequently brought up by politically motivated critics of the research is that there is still no single identified "gay gene." However, the same is true for height, skin color, handedness, frequency of heart disease and many other traits that have a large inherited component but no dominant gene. This doesn't mean that sexual orientation is a choice; it simply confirms that sexual orientation is complex, with many genes contributing to the phenotype...

There is good reason for their opposition to the scientific findings. Studies in college classrooms have shown that exposure of students to information about the causes of sexual orientation has a direct, positive influence on their opinions about LGBT civil rights. This fits with polling data showing that people who believe that gays are "born that way" are generally supportive of full equality, whereas more than two thirds of those who believe it is "a choice" are so opposed that they favor the re-criminalization of same-sex relations.

....t it is essential to acknowledge that lack of scientific knowledge can actually result in having our rights and freedoms taken away through the actions of misinformed voters, legislators and judges.

More about homosexuality in animals

There's an outstanding piece in the New York Times Magazine coming this weekend on homosexuality in the animal world. It's a thoughtful discussion on how much we try to anthropomorphize animals, inapprorpriately. What it is to be homosexual or same-sex paired as an animal may vary by species and population and not be at all comparable to what it means to be a GLBT human. But the fact is that many animals do show same sex pairing.
Excellent article!

In the course of her doctoral work, Young and a colleague discovered, almost incidentally, that a third of the pairs at Kaena Point actually consisted of two female birds, not one male and one female. Laysan albatrosses are one of countless species in which the two sexes look basically identical. It turned out that many of the female-female pairs, at Kaena Point and at a colony that Young’s colleague studied on Kauai, had been together for 4, 8 or even 19 years — as far back as the biologists’ data went, in some cases. The female-female pairs had been incubating eggs together, rearing chicks and just generally passing under everybody’s nose for what you might call “straight” couples....

In recent years though, more biologists have been looking objectively at same-sex sexuality in animals — approaching it as real science. For Young, the existence of so many female-female albatross pairs disproved assumptions that she didn’t even realize she’d been making and, in the process, raised a chain of progressively more complicated questions. One of the prickliest, it seemed, was how a scientist is even supposed to talk about any of this, given how eager the rest of us have been to twist the sex lives of animals into allegories of our own.
Update: here's an updated link if the first one doesn't work

More information
For other general audience discussions of the science of orientation, there's a good article in NY magazine: The science of gaydar.

Here's a link to the New Scientist, explaining why homosexuality is not incompatible with principles of natural selection.

And an article at the HuffPo, summarizing the increasing evidence that there are biological differences between gay and straight people.

Recent studies:
Identical twins may not be completely genetically identical (Apr 2008)
Epigenetic modifications linked to being gay (Dec 2012)
and again, Epigenetic modifications linked to being gay (Oct 2015) although this study has been challenged for bad statistics and too small a sample size

My view from a conversation originally posted on another blog.

No professional geneticist would consider it likely to be an absolute correlation in twin studies for a complex, almost certainly multi-genic trait such as sexuality. 30% is very high correlation. There is a genetic component to sexuality; but as with anything else, genetics is not absolute. Mendel's laws are accurate regarding the heritability of different, individual and unlinked genes (ie., genotype) but expression and penetrance can vary enormously (ie, phenotype).

As well as stochastic (e.g., by chance) variation, there is also epigenetic modification that can reflect environmental cues. For example, if you take genetically identical rat pulps and have them raised by foster mothers who provide different degrees of care, you can change the expression of stress genes in the pups depending on the degree of care they received. (See here.)  However, if the pups already had a particular allele, or genetic variant in that gene, that could supersede the effects of the nurture: to be, in a sense, epistatic over the epigenetic.

Complex traits are a mixture of genetics, environment, and chance. Only a very naive person would believe there needs to be a single "gay gene" to associate with such a complex behavioral trait.

Now I knw someone would bring up alcoholism. Isn't the question really one of where we draw the line between a normal human variant, and a pathology? We would not today try to "cure" left-handedness, although at one time we did, with awful effects. If you look at the Deaf culture, many Deaf people do not consider themselves in need of a cure, but rather see themselves as a normal variant. Yet most of us who hear, would think that being Deaf is a defect. Still, while we might concede that, we would all agree that alcoholism is a pathology. However, you cannot call being gay a "pathology" just because it is a variant, any more than a red-head is a pathology just because they have a variant allele in the melanocortinin receptor.

Can you say being gay is "harmful"? Not really. Most studies that claim that gay people die young are out-dated or inaccurate. To go on and claim it is, is also to deny the witness and experience of many healthy and happy LGBT people. I have a strong suspicion that most of those railing about being gay being a "choice" do not actually know --really KNOW-- any gay people.

Medical and psychological experts agree . Chalk it all up to politics you may, but that seems to be the fundamentalist view. Denying the consensus of science and medicine on this is about as accurate as denying heliocentrism.

You are right that it isn't a defined "light switch". Indeed, almost no human traits are. Even something as apparently simple as ABO bloodtype is modified by other genes (see, for example, the Bombay Phenotype). Sexuality is complex, probably expressed on a gradient in keeping with the complexity of its causes. Many gay people knew themselves to be gay very young. Others are more fluid in their sexuality. However, it's not an absolute: to say that traits are complex, and not simply "on/off" is not to say there is no genetic component.

Take, for example, domestic sheep, in which about 5-8% of rams are homosexual, and 5-8% are asexual. (aside, no one has addressed the fact that some people are asexual too!) Despite years of effort in trying to "breed out" those traits, they yet persist.

Genetics is like a palette that sets the stage. My genes may give me the potential to be tall. However, if I am not fed properly as a youth, I will not be tall regardless of my genes. And incidentally, although we would surely agree that height has a genetic component, there is no single "Tall" gene. Why would we expect any different from sexuality?

Willfully ignoring the facts of biology is IMHO more unhelpful to the debate. Significantly, it also denies and disrespects the experience of individual gay people, many of whom knew very young that they were gay, just as you at some point knew for sure that you wouldn't. To me, it's far more productive to discuss why a complex, variant human behavior is viewed by some as a normal variant and others as a pathology. It would be helpful if people did not resort to pseudo-science to obfuscate that discussion, and to dehumanize and devalue the individuals who have that trait.

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