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Studies of the causes of homosexual orientation

More detailed coverage of 2 studies on
multiple genes, and brain structure.

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This topic is continued from an earlier essay

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Study 8: Based on multiple genetic origins to male homosexuality:

The Hamer study, described previously, showed that the Xq28 region of the X chromosome can cause a propensity towards homosexuality. Brian Mustakski of the University of Illinois in Chicago and colleagues at the National Institutes of Health searched for additional genes among other chromosomes which might be also involved in causing male homosexuality.

Their paper was published in the peer reviewed Human Genetics Journal. Two articles which analyzed the paper reached opposite conclusions:

  • One article, published in LifeSite, a conservative Christian web site, concluded that Mustakski was unable to prove the involvement of other chromosomes. This supported their belief that genes are not involved in homosexuality. 1

  • The other article, published in GayLife, a lesbian, gay, bisexual, and transgender [LGBT] support group, reached the opposite conclusion: that genes are involved in homosexuality. 2

Both articles were correct in what they report. However, they interpret the results to confirm their pre-existing beliefs about the cause of homosexuality.

Dr. Mustaksi remarked:

"It's the largest molecular genetic study ever done on sexual orientation. ... There is no one 'gay' gene. Sexual orientation is a complex trait, so it's not surprising that we found several DNA regions involved in its expression."
They found three such locations. They studied the DNA of persons from 146 families in which there were at least two gay brothers. They included brothers from the 40 families use in the Hamer study of 1993, as well as from 106 new families.

Brothers, who are not identical twins, share about 50% of their genes. The study searched for markers -- short strips of DNA that commonly correspond to specific parts of a chromosome. If all of the brothers in the study shared any specific marker significantly more than 50% of the time, then the researchers would have found a genetic link to homosexual orientation.

They detected three chromosomal regions on the human genome more than 50 percent of the time; they were on chromosomes 7, 8, and 10. Of these, a region 7q36 on chromosome 7 was shared most frequently. Since males "inherited this region from their fathers just as often as their mothers, a finding that suggests genes from both parents can contribute to a son's sexual orientation." 1

As in previous studies, they were unable to confirm the results of Hamer's discovery of the Xq28 region of the X chromosome as a cause of homosexuality among their 146 test subjects. However, when they retested the 40 pairs that Hamer had used, they found that Xq28 could contain genes linked to sexual orientation. Mustanski refers to this as "locus heterogeneity" '-- a situation in which one gene might influence sexual orientation in one family, but not in another family. Recall that Hamer's study selected only gays who had a surplus of gays in their mother's side of the family.

Dr. Mustanski said:

"The fact that we found suggestive evidence of different areas where there might be genes for sexual orientation builds upon the research that's been done on family studies, twin studies, and previous molecular genetic studies which are consistently showing evidence of genetic influences."

The chromosome 7 finding may shed new life on a Dutch study. A gene near 7q36 is known to be responsible for the creation of a brain center called the suprachiasmatic nucleus. A Dutch research team in 1990 found that men with a homosexual orientation have a larger suprachiasmatic nucleus than heterosexual men. Mustanski said:

"Perhaps this gene results in different brain structure and that brain structure causes someone to be heterosexual or homosexual."

Dr. Richard Pillard, a psychiatrist at Boston University specializing in  sexual orientation suggests that Mustanki's study should be repeated with a larger number of genetic markers. He said that this study:

"Doesn't come up with a definitive answer. The first step is to say, 'yes there's a genetic contribution.' The next step is to say, 'well how are these genes expressed in the brain, as we presume they must be'?"

Dr. Mustanski concluded that:

"There's a converging line of evidence between the hormonal studies, the genetic studies, and the neuroanatomical studies. My research has identified candidate genes within these new chromosomal regions that could link together all of these different findings." 1

Warren Thockmorton and Ray Durwood are professors at the Grove City College -- a conservative Christian college which rejects "relativism and secularism." 3 They reviewed the Mustanki study and said:

"The authors describe in the article three non-X chromosomal 'new regions of genetic interest' (7q36, 8p12, and 10q26). In the authors' view, a noteworthy aspect of the study as follows: 'Our strongest finding was on 7q36 with a combined mlod score of 3.45 and equal distribution from maternal and paternal allele transmission. This score falls just short of Lander and Kruglyak's (1995) criteria for genomewide significance.' They go on to say 'two additional regions (8p12 and 10q26) approached the criteria for suggestive linkage' - again pointing out that neither was statistically significant." 2

If one assumes that the propensity towards homosexual orientation is determined by an interaction of many genes, then no region on any one chromosome can be expected to be conclusively proven. The finding that the chromosome 7 was very close to significant probably shows that it must be significant in some male homosexuals.

It is worth noting that most studies into the causes of homosexuality seem to show that about 10% of individuals are set up to be potentially homosexual by some gene or combination of genes. However, only a little over half of them are triggered by an unknown environmental factor during early childhood so that they will develop as homosexuals after puberty. The Mustanski study shows that, if this theory is correct, some combination of multiple genes is involved, rather than a single gene.

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Study 9: LaVay study of brain structure:

Simon LeVay, a Neuroanatomist at the Salk Institute in California published a study in 1991 which examined the brains of men, many of whom had died of AIDS. He found that the INAH 3 (a structure within the hypothalamus) differed in size between heterosexual and homosexual men. This suggested to the researcher that "sexual orientation has a biological substrate". 4

This study has been criticized on a number of grounds. Many of his subjects died of AIDS; perhaps the difference in structure size was caused by the disease, or the medications that they took, rather than the person's sexual orientation. Others have suggested that homosexual or heterosexual activity may have determined the size of the structure, rather than vice-versa.

It would seem that the study proves little. However, it appears to be frequently cited in books and on the Internet.

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More studies are described in the next essay

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References used:

The following information sources were used to prepare and update the above essay. The hyperlinks are not necessarily still active today.

  1. Ariel Whitworth, "Studies Suggest Multiple Genes Contribute to Sexual Orientation," Gay Life, 2005-FEB-18, at: http://www.bgp.org/
  2. "New Genetics Study Undermines Gay Gene Theory," LifeSite, 2005-FEB-10, at: http://www.lifesite.net/
  3. "The Mission of Grove City College," at: http://www.gcc.edu/
  4. Simon LeVay, "The Sexual Brain," MIT Press, (1994). Read reviews or order this book safely from Amazon.com online book store
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Copyright © 1997 to 2013 by Ontario Consultants on Religious Tolerance
Latest update: 2008-JAN-20
Author: B.A. Robinson

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