Epigenetics is the study of how gene expression may be regulated by temporary DNA switches called "epi-marks."
For decades, there have been indications that homosexual orientation is determined before birth. The most convincing proof involves studies of twins.
If a set of twins is identical, then their DNA will be identical because they both originate from a single ovum and spermatozoon that splits in two after conception. Studies have shown that If one identical twin is gay, then the chances of the other twin also being gay is about 20% to 50% (sources differ). 1
If a set of twins is fraternal, then their DNA will be different because they originate from two separate spermatozoa fertilizing two separate ova. Studies have shown that if one fraternal twin is gay the chances of the other being gay is about 5%. This is about the occurrence in the general population.
Although this indicates that DNA is somehow involved in homosexual and bisexual orientation, the precise mechanism has remained a mystery. 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 be the direct cause of the three sexual orientations: homosexual, bisexual, and heterosexual. It would appear that a simple "gay gene" or set of "gay genes" that directly cause homosexuality may never be found because they may not exist.
Many commentators believe that homosexuality is determined after birth, either because of some factor in a persons upbringing or because of having been sexually assaulted at a young age. They have noted that 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 who are genetically related to them. Bisexuals are attracted to both the same and opposite sex, but not necessarily to the same degree. Thus both homosexuals and bisexuals tend to procreate less. If homosexuality is a trait with a purely genetic/DNA cause, one would expect that it would become very rare within a given population group in a few generations. Heterosexuals would simply outbreed homosexuals and bisexuals. But every society on earth appears to have a more or less a stable and similar percentage of gays and lesbians from generation to generation. Some believed that the theory of natural selection proved that homosexuality is determined from something in the environment after birth. However, the natural selection argument was weakened or eliminated when it was found that female relatives of homosexuals tend to have more children than average. That is, the same genetically-related cause that produces homosexuality in some people also appears to increase fecundity in female relatives. This increased the support that homosexuality has some type of genetic cause. But the exact cause remained a mystery because:
DNA may not be directly involved, or scientists would probably have been able to isolate the gene or group of gene causing homosexual orientation.
Yet the cause seems to be related to DNA because that is the only type of link that can effect both individuals and their female relatives.
2012: A new possible explanation for the cause of homosexuality was unveiled:
Researchers at the Working Group on Intragenomic Conflict at the National Institute for Mathematical and Biological Synthesis (NIMBioS) may have solved the mystery. They issued a report on epigenetics. Associated with the report is 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 and the occurrences of intersexuality!
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." 2
According to a NIMBios article:
"Epi-marks constitute an extra layer of information attached to our genes' backbones that regulates ...expression [of the genes]. 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." 3
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 ..." 3 affect the gender(s) to which the individual is sexually attracted later in life. That is, they also determine the person's sexual orientation. If this is true, then epi-marks may play a role in intersexuality, gender identity, and sexual orientation.
An inexact parallel would involve a computer. The DNA corresponds to a computer program while the epi-marks correspond to the operating system.
If there is a connection between epi-marks and homosexuality, it would be a remarkable development. It would prove some beliefs of many religious conservatives to be invalid. For example, that:
Sexual orientation is chosen by the individual and can be readily changed with prayer and counseling. If the epigenetic model turns out to be correct, then sexual orientation would be proven to be established before birth, and discovered later in life. It would probably be rarely or never changeable.
Gender identity is caused by mental confusion. If the epigenetic model turns out to be correct, then gender identity would be determined by one's internal brain structure as caused by the epi-marks.
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, (with XX sex chromosomes) the epi-marks prevent her from becoming masculinized during intervals of high testosterone. For male fetuses, (typically with XY sex chromosomes) the epi-marks 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 passed on 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 allowing him to be feminized later in gestation.
In both cases, the normal effects of the epi-marks would be reversed.
Gavrilets explained in a Quarterly Review of Biology article:
"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." 4
He is apparently using the term "theory" in its popular sense to refer to a hypothesis, speculation, assumption, or presupposition. Usually, in science, the term is used to refer to a hypothesis that has been thoroughly tested, is proven to be accurate, and widely assumed by scientists to be true.
If an epi-mark inverts the gender to which the fetus will be attracted later in life, then a newborn may result who discover that she or he is gay, lesbian, or bisexual later in life. Similarly, if an epi-mark inverts the gender by which the fetus will identify themselves later in life, then baby many be born who will eventually be transgender.
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 2012 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." 5