Dara Newman Biology 103 WWW report Nov. 24, 1997
The Genetics of Homosexuality
In trying to decide on a topic for this WWW project, it seemed logical to try and focus on a current subject. Homosexuality and homosexual behavior has existed for thousands and thousands of years, probably even before the times of homo-sapiens. However, up until a few years ago, the issue was discussed mostly by people in the social sciences. Psychologists, such as Freud, studied homosexuals extensively in hopes of coming up with an explanation for their "abnormal" behavior. All of the explanations that these people created linked homosexuality to experiences that homosexuals have while growing up. Generally speaking, people in the world of psychology believed that homosexuality could be explained by a person's environment. However, in the past four or five years, the subject of homosexuality has been creeping into the world of biology. Studies have been done recently that attempt to look at homosexuality in a scientific light in hopes of coming up with a genetic explanation for sexual preference.
One of the first successful scientific studies that was done on homosexuality was reported on in 1993. The purpose of this study was to look at families in which there was an abnormally high occurrence of homosexuality. By extensively studying the family histories of these families, researchers hoped to find some clues pointing towards the genetic factors that affect homosexuality. That is exactly what happened. By looking at the family trees of gay males (For some reason, this study only focused on male homosexuality, but made the claim that their findings would be similar to the ones that would be found by looking at female homosexuality. As this paper will discuss later, this assumption that male and female homosexuality can easily be compared may be entirely inaccurate.) it seemed that the majority of homosexual occurrences were on the maternal side of the tree. From this information, researchers concluded that if in fact there was a "homosexual gene," it appeared to be passed down from mother to son. This means that heterosexual females are carriers of this gene, and when it is passed down to a male child, there is a chance that the child will be a homosexual. While this study did not come up with any hard core facts about the genetics of homosexuality, it showed that a connection very well could exist. Since this study did determine that the gene influencing homosexuality was carried by the mother, researchers participating in further studies knew that they could limit their search to the X chromosome, and that is exactly what they did (5).
One of the most influential studies on the genetics of homosexuality was done by Dean Hamer and his co-workers at the National Cancer Institute in Washington DC (1993). Hamer's research involved studying thirty-two pairs of brothers who were either "exclusively or mostly" homosexual. None of the sets of brothers were related. Of the thirty-two pairs, Hamer and his colleagues found that two-thirds of them (twenty-two of the sets of brothers) shared the same type of genetic material. This strongly supports the hypothesis that there is an existing gene that influences homosexuality (4). Hamer then looked closely at the DNA of these gay brothers to try and find the region of the X chromosome (since the earlier research suggested that the gene was passed down maternally) that most of the homosexual brothers shared. He discovered that homosexual brothers have a much higher likelihood of inheriting the same genetic sequence on the region of the X chromosome identified by Xq28, than heterosexual brothers of the same gay men. Keep in mind though, that this is just a region of the X chromosome, not a specific gene. Although researchers are hopeful, a single gene has not yet been identified (7). Hamer's study also acknowledges the fact that while it does suggest that there is a gene that influences homosexuality, it has not yet been determined how greatly the gene influences whether or not a person will be homosexual (4). In addition, Hamer attempted to locate a similar gene in female homosexuals, but was unsuccessful (7). The results that Hamer's study did find though, cannot yet be accepted as absolute truth. Another study took place in 1993 by Macke et al. This study examined the same gene locus as the Hamer study, but found that it had no influence on homosexuality (8). As you can see, the results on this topic are still extremely varied and reasonably new, so it is difficult to come to any lasting conclusion.
Other studies have been conducted that look at twin brothers rather than brothers of different ages. Bailey and Pillard (1991) did a study of twins that determined a "52% concordance of homosexuality in monozygotic twins, 22% for dizygotic twins, and 11% for adoptive brothers of homosexual men" (8). These results, like Hamer's, provide further support for the claim that homosexuality is genetically linked. Studies very similar to the Bailey and Pillard study have been done both with female homosexual siblings and siblings of both sexes. The results for both of these studies were only off from Bailey and Pillard's by a few percentage points. Putting all of these results together, it seems like genetics are at least 50% accountable for determining a person's sexual orientation (8).
Looking at the results of many of the other studies I have discussed, it seems a little strange to me that the student of homosexual siblings who were both male and female came up with similar result as the studies that looked exclusively at male homosexuality. Hamer's study, along with others, have tried to located a gene that influences female homosexuality, but they have been unsuccessful. More importantly, the region of the X chromosome that very possibly could influence male homosexuality does not influence females in the same way. Female heterosexuals merely pass the gene sequence on to their sons. Knowing this, it seems odd to me that there would be such a high percentage of male and female homosexual siblings. Perhaps this suggests that if genetics are responsible for homosexuality, we have a long way to go before we completely understand the gene locus' that determine sexuality.
Aside from the scientists who are researching the topic of homosexuality and genetics, there are many other people who have concerns and vested interests in the topic. The information that is being discovered has been used by people in both positive and negative ways. On the one hand, there are members of the gay community who are very excited to find that the life-style they live is not entirely a choice that they made, as homophobic people often like to believe. Some homosexuals feel that if the world realizes that homosexuality is something people are born with, just like the color of your skin or your eyes, then people will begin to be more accepting of the homosexual life-style (5). However, on the other hand, there is also a group a people who believe that if homosexuality is in fact genetically linked, then there should be a way to genetically alter homosexuals in order to make them "normal" (3).
Before I started researching this topic on the world-wide-web, I did not realize what a new and controversial issue the genetics of homosexuality was. From tid-bits of news that I had picked up along the way, I thought that scientists had located, without a doubt, a gene that plays a role in influencing sexual orientation. From the research that I have discussed above, that is obviously not the case. I am eager to follow this subject more in the future and see what biology will discover next.
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This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.