The cause(s) of sexual orientation: Nature vs. Nurture
Epigenetics: A promising theory that homosexuality
and bisexuality have a hereditary component
This topic "The cause(s) of sexual orientation" is continued from an earlier essay
Study 17: Epigenetics: Epigenetics is the study of how gene expression may be regulated by temporary DNA switches called "epi-marks."
For decades, the root cause of homosexual and bisexual orientation has remained a mystery. Although many recent studies have given strong indications that the cause has a hereditary component, there remain some gaps in the scientific understanding of these two sexual orientations:
- No smoking gun has been found so far. In spite of massive research efforts, no gene or group of genes has been definitively proven to cause homosexuality.
- The Darwinian theory of natural selection is based on the survival of the "fittest" where fittest is defined as those members of a species that have the largest number of offspring. They are the individuals that most successfully pass their genes on to the next generation. Gays and lesbians, by definition, are not sexually attracted to members of the opposite sex and thus tend to have many fewer offspring. Bisexuals are attracted to the opposite sex to some degree, but are also attracted to the same sex. Thus both homosexuals and bisexuals tend to procreate less. If homosexuality is a trait with a purely genetic cause, one would expect that it would become very rare within a given population group in a few generations. Heterosexuals simply outbreed homosexuals and bisexuals. But every society on earth has a more or less a stable percentage of gays and lesbians. It would appear that a simple "gay gene" or set of "gay genes" may never be found because they may not exist.
Researchers at the Working Group on Intragenomic Conflict at the National Institute for Mathematical and Biological Synthesis (NIMBioS) have issued a report on epigenetics. They have produced a mathematical model that shows how epi-marks before birth might play a major role in determining a person's sexual orientation and perhaps even gender identity.
NIMBioS researcher Sergey Gavrilets explained:
"It’s not genetics. It’s not DNA. It’s not pieces of DNA. It’s epigenetics. The hypothesis we put forward is based on epigenetic marks." 1
According to a NIMBios article:
"Epi-marks constitute an extra layer of information attached to our genes' backbones that regulates ...[the genes'] expression. While genes hold the instructions, epi-marks direct how those instructions are carried out -- when, where and how much a gene is expressed during development." 2
These epi-marks are normally specific to the gender of the fetus. They are produced early in gestation, during the embryonic stage of development. The speculation is that some epi-marks "... affect the genitals, others sexual identity, and yet others ..." 2 affect the gender(s) to which the individual is sexually attracted later in life -- their sexual orientation. If this is true, then epi-marks may play a role in intersexuality, gender identity and sexual orientation.
Certain epi-marks become active later in pregnancy when they control the reaction of the fetus to normal fluctuations in testosterone levels. In the case of a female fetus, (XX) the epi-marks prevent her from becoming masculinized during intervals of high testosterone. For male fetuses, (XY) they prevent him from becoming feminized during intervals of low testosterone.
Normally, these epi-marks are eventually inactivated or "erased" during conception. They are not typically present to be transmitted from generation to generation. However, rarely, it is possible for these particular epi-marks to be transmitted at conception:
- from the father to a girl embryo, thus allowing her to be masculinized later in gestation, or
- from the mother to a male embryo, thus feminizing him later in gestation.
In both cases, the normal effects of the epi-marks would be inverted.
"These models that we develop will work for both gays and lesbians. I don't think there's any other theory that works for both. The existing research focuses more on gay men." 6
If an epi-mark inverts the gender to which the fetus will be attracted later in life, then a gay. lesbian or bisexual newborn may result. Similarly, if an epi-mark inverts the gender with which the fetus will identify themselves later in life, then a transgender baby many be born.
Co-author of the study, Sergey Gavrilets, said that this relatively rare:
"Transmission of sexually antagonistic epi-marks between generations is the most plausible evolutionary mechanism of the phenomenon of human homosexuality."
The report's abstract says in part:
"Our model predicts that homosexuality is part of a wider phenomenon in which recently evolved androgen-influenced traits commonly display gonad-trait discordances at substantial frequency, and that the molecular feature underlying most homosexuality is not DNA polymorphism(s), but epi-marks that evolved to canalize sexual dimorphic development that sometimes carryover across generations and contribute to gonad-trait discordances in opposite-sex descendants." 3
The group defines "homosexuality" to include the full range of homosexual and bisexual orientation.
At this time, epi-marks are still an untested theory. However, it neatly fits with many observations and studies including:
- the perseverence of homosexuality in spite of evolutionary forces which would normally tend to eradicate it,
- the 16 or so studies of homosexuality that all apparently link sexual orientation to pre-birth experience, and
- the inability of researchers to find a gay gene or genes.
William Rice, the study’s lead author, said:
"We've found a story that looks really good.This can be tested and proven within six months. It's easy to test. If it's a bad idea, we can throw it away in short order." 4
According to Nathan Bailey, an evolutionary biologist at the University of St Andrews in Scotland who was not involved in the research:
"The study provides a very interesting, but as yet untested, genetic mechanism for the evolutionary maintenance of human homosexuality. We are going to have to wait until more evidence is in, but I do think it would be exciting to know whether epi-marks contribute to the expression of sexual orientation in humans." 5
Reaction to this new theory is mixed:
- Some religious conservatives are unhappy because they promote family dysfunction and childhood sexual molestation as the causes of homosexuality. The epi-mark theory continues the search for a pre-natal cause for homosexuality with a hereditary component.
- Some in the lesbian, gay and bisexual community are unhappy because they feel that a scientific investigation is not necessary to justify who they are.
- Some in the scientific community have expressed strong interest in conducting experiements that will test the theory. 1
The following information sources were used to prepare and update the above
essay. The hyperlinks are not necessarily still active today.
- Laura Blue, "New Insight into the (Epi)Genetic Roots of Homosexuality, Time, 2012-DEC-13, at: http://healthland.time.com/
- "Study Finds Epigenetics, Not Genetics, Underlies Homosexuality," NIMBioS, 2012-DEC-11, at: http://www.nimbios.org/
- "Homosexuality as a Consequence of Epigenetically Canalized Sexual Development," Quarterly Review of Biology, Vol. 87, #4, at: http://www.jstor.org/
- Christine Roberts, "Scientists uncover possible source of homosexuality," New York Daily News, 2012-DEC-12, at: http://www.nydailynews.com/
- Randy Astaiza, "NEW THEORY: The Gay Trait Is Passed Down From Parent To Child," Business Insider, 2012-DEC-14, at: http://www.businessinsider.com/
- "Study suggests homosexuality may be inherited," 11 Alive, 2012-DEC-12, at: http://www.11alive.com/
Copyright © 2013 by Ontario Consultants on Religious Tolerance
Latest update: 2013-JAN-22
Author: B.A. Robinson