|MadSci Network: Genetics|
I answered this question before, but on a higher level. It is a very tough question to answer.
Research towards your question is hard. First of all, is it a disease? Most people won't think so (except the maybe sick-making 'Ellen-coming-out' event). However, there is some research which points to some genes which may be involved. It can also have something to do with the environment in which people are raised. Many people may be gay but don't wish to express it, which will give bad results in large scale tests.
In the articles I've read, I think the relationships between being-gay and certain genes are not very strong. When being-gay is genetic, many genes will probably be involved (as is often the case).
Personally, I don't see homosexuality as a disease, so I wouldn't call the differences in genes defects.
In summary, the answer is " I DON'T KNOW, YET". Being homosexual may be genetic, it may have something to do with the environment in which someone is raised, but most likely a combination of many genes and environment.
It will take many many more years before a definite answer can be given.
Additional commentary by Mad Scientist Network volunteer Louise Freeman:
Some time ago, I was sent your question to answer, and unfortunately, it was a while before I could get to it. In the meantime, I saw that someone else has answered your question on the site. I had a few things to add to Rolf's answer.
You ask a question that is a very hot topic among scientists right now. The short answer to your question is "Probably yes, at least partially." There are several studies that provide evidence that homosexuality has a partial genetic basis.
The first type of study involves pedigree analysis, or examining the families of gay men to see if they have close relatives who are also gay. On the surface, this might not seem to prove anything... even if you could show that being gay "runs in the family" it might be due to environment rather than genes. To draw an analogy, if three brothers all suffered from heart disease, it could be that they all inherited a gene that predisposed them to this condition, or it could be something in the family environment... maybe they were raised with a high-cholesterol diet, for instance, and continued to eat that way for most of their lives, which resulted in all of them having heart attacks! But researchers doing pedigree analysis found that gay men tended to have other gay men for uncles, but usually on their mother's rather than their father's side. This lend researchers to believe that a gene on the X-chromosome might be linked to homosexuality.... more on this later!
The second piece of evidence comes from twin studies. Identical twins, as you probably know, share all of their genes. Non-identical, or fraternal twins share half their genes... the same rate as other brothers and sisters. Finally, adopted kids share no genetic material, but are raised in the same environment from an early age. A common way of looking for genetic link for any trait is to compare the concordance rate in such pairs of siblings. Concordance rate is the chance that the siblings share a trait. If you and your brother both have brown eyes, you are concordant for that trait. If you have brown eyes and he has blue, you are discordant for eye color. The concordance rate for hair color (assuming neither twin dyes it) in identical twins is near 100%... identical twins have the same hair color because hair color is determined by your genes. Non-identical twins are less likely to have the same color hair... so they have a lower concordance rate. Adopted siblings' concordance rate for hair color is even lower. This is not to say that non-identical, or adopted siblings never have the same hair color... of course, some do, but the _percentage_ of those that are concordant for that trait is much less than 100%.
On the other hand, if the concordance rate for a trait is the same in genetic and adopted pairs of siblings, it is likely that genes play little or no role, but that the trait is caused by something in the environment that the siblings share, such as family upbringing. For instance, children growing up in the same home almost always learn the same launguage. The concordance rate for the language they speak as adults is therefore also near 100%, no matter if they are identical twins, nonidentical twins, or adopted siblings (assuming they were adopted as babies, before they learned to talk!). Genes, as you might guess, do not influence what language you speak; that is entirely up to what language you learn as a baby! Most behavioral traits, however, are not due to just genes or just environment, but to a combination of both.
Researchers found that the concordance rate for homosexuality is highest among identical twins, lower among fraternal and lowest in adopted siblings. Such a pattern is evidence for a genetic basis to homosexuality... the closer you are genetically to your sibling, the more likely you both are to be gay. If you are not genetically related, you are less likley to both be gay, even though you grew up in the same home. However, it should also be stressed that the researchers found aconcordance rate of around 50% for both gay and lesbian pairs of identical twins... *not* 100%. In other words, half of those with an identical twin who is gay are also gay, half are not. This probably means that, although genes play a role in sexual orientation, they are likely *not* to be the only factor, as they are for traits like eye and hair color.
The final piece of evidence comes from studies on gene linkage. Remember the pedigree analysis that suggested a gene for male homosexuality on the X-chromosome. To review some basic genetics, girls inherit 2 X-chromosomes, one from each parent. Boys get an X-chromosome from their mother, and a Y-chromosome from their father. The fact that gay men tended to have more gay relatives on the mother's side than on the father's suggested that, if homosexuality was inherited, the gene would be on the X-chromosome.
In 1993, a scientist named Hamer studies 40 pairs of homosexual brothers who were not twins. He found the same set of chromosomal markers on one part of the X chromosomes of 33 pairs of the gay brothers... a much higher rate than could be explained by mere coincidence. It is therefore possible that this region of the X chromosome contains a gene that predisposes a male to be gay. However, as in the twin study, it is unlikely that the gene could explain all aspects of homosexuality. Seven of the pairs of brothers did not share these markers, meaning that something else, genetic or environmental, must cause their homosexuality. Also, we do not know how many heterosexual brothers of these men also carried the same genetic markers. (Hamer only looked at the chromosomes of brothers who identified themselves as gay. As Rolf pointed out, some men with homosexual tendencies repress or lie about them because of societal pressures. Hamer assumed the men who said they were gay were not likely to be misrepresenting their sexual orientation.) So, while there is some evidence that homosexuality has a partial genetic basis, it is far from conclusive, and environmental factors almost certainly play a role, as well. Furthermore, no one knows how a "gay gene", assuming it exists, works to make a man be sexually attracted to other men.
As far as I know, although the twin studies suggest a genetic basis for homosexuality in women, linkage studies have not yet been done in lesbian sisters.
More studies on the link between genetics and sexual orientation are underway, and the issue will likely be debated for years to come. Even if a genetic link to homosexuality is conclusively proven, it is unlikely to settle the debate over homosexual rights in our society. Homosexuality could be seen as a natural and harmless variation, such as being born left-handed, or it could be viewed as a genetic disease that should be "cured." Scientists can seek to understand the causes of homosexuality, whether they be genetic or environmental, but it is up to society to decide how that information will be interpreted and used.
Further information on the search for a genetic link to homosexuality, and the debate that search has caused among scientists and nonscientists can be found at members.aol.com/gaygene/index.htm.
The American Psychological Association also has information on homosexuality. Go to Psychcrawler and type "homosexuality" as a search term. Thank you for your question, and I'm sorry it took me so long to reply.
Louise M. Freeman, PhD.
University of Virginia Biology Department
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