Bill Sullivan, Professor of Pharmacology and Toxicology, Indiana University
September 3, 2019 9:40 am EDT
In all cultures, 2% to 10% of people report having same-sex relationships. In the US, 1% to 2.2% of women and men, respectively, identify as gay. Despite these numbers, many people still consider same-sex behavior an abnormal choice. However, biologists have documented same-sex behavior in more than 450 species, arguing that same-sex behavior is not an unnatural choice and may, in fact, play a vital role within populations.
In a recent issue of the journal Science, geneticist Andrea Ganna of the Broad Institute of MIT and Harvard, and her colleagues, describe the largest survey to date of genes associated with same-sex behavior. By analyzing the DNA of nearly half a million people in the United States and the United Kingdom, they concluded that genes account for between 8% and 25% of same-sex behavior.
Numerous studies have established that sex is not simply male or female. Rather, it is a continuum that emerges from a person's genetic makeup. Nevertheless, misconceptions persist that same-sex attraction is a choice that justifies condemnation or conversion, and leads to discrimination and persecution.
I am a molecular biologist and am interested in this new study because it further illuminates the genetic contribution to human behavior. As the author of the book, "Nice to Meet Me: Genes, Germs, and the Curious Forces That Make Us Who We Are," I have conducted extensive research on the biological forces that conspire to shape human personality and behavior, including factors influencing sexual attraction.
The search for 'gay genes'
The new finding is consistent with multiple previous twin studies that indicated same-sex attraction is a heritable trait.
A new study suggests that genes are responsible for between 8% and 25% of same-sex preferences. Guru 3D
The new study is the latest in a search for “gay genes,” which began in 1993 when Dean Hamer linked male homosexuality to a section of the X chromosome. As genome sequencing became easier and more affordable, additional genetic candidates emerged with potential links to homosexual behavior. So-called genome-wide association studies identified a gene called SLITRK6, which is active in a brain region called the diencephalon that differs in size between gay and straight people.
Genetic studies in mice have uncovered additional genetic candidates that could influence sexual preference. A 2010 study linked sexual preference to a gene called fucose mutarotase. When the gene was deleted in female mice, they were attracted to female odors and preferred to mount females rather than males.
Other studies have shown that disrupting a gene called TRPC2 can cause female mice to behave like males. Male mice lacking TRPC2 no longer exhibit male-to-male aggression and initiate sexual behavior with both males and females. Expressed in the brain, TRPC2 functions in the recognition of pheromones, chemicals released by one member of a species to elicit a response in another.
With multiple genetic candidates linked to homosexuality, it seemed highly unlikely that a single "gay" gene existed. This idea is supported by the new study, which identified five new genetic loci (fixed positions on chromosomes) that correlate with same-sex activity: two that appeared in both men and women, two only in men, and one only in women.
How might these genes influence same-sex behavior?
I find it intriguing that some of the genes in men identified in Ganna's study are associated with olfactory systems, a finding that has parallels with work in mice. Ganna's group found other genetic variants that may be related to the regulation of sex hormones, which other scientists have previously suggested play an important role in shaping the brain in ways that influence sexual behavior.
It is believed that conditions in the womb during pregnancy influence a child's sexual orientation. Anna Om / Shutterstock.com
Men with a genetic condition called androgen insensitivity syndrome can develop female genitalia and are typically raised as girls, despite being genetically male, with one X and one Y chromosome, and being attracted to men. This suggests that testosterone is needed to "masculinize" a prenatal brain; if that doesn't happen, the child will grow up to desire men.
Similarly, girls who have a genetic condition called congenital adrenal hyperplasia are exposed to unusually high levels of male hormones such as testosterone while in the womb, which can masculinize their brains and increase the likelihood of lesbianism.
It is also possible that hormonal changes during pregnancy affect the development of the fetal brain. In rats, manipulating hormones during pregnancy produces offspring that exhibit homosexual behavior.
Why does homosexual behavior exist?
Several hypotheses have been put forward to explain how homosexuality might be beneficial in perpetuating family genes. One idea involves the concept of kin selection, whereby people work to ensure the passage of their family's genes to subsequent generations. Gay uncles and aunts, for example, are "helpers in the nest" who help raise the children of other family members to care for the family tree.
Another idea suggests that homosexuality is a "compensatory trait." For example, certain genes in women help increase their fertility, but if these genes are expressed in a man, they predispose him to homosexuality.
Sexual behavior is highly diverse and governed by sophisticated mechanisms throughout the animal kingdom. As with other complex behaviors, it is not possible to predict sexuality by observing a DNA sequence as if it were a crystal ball. Such behaviors arise from constellations of hundreds, perhaps thousands, of genes and how they are regulated by the environment.
While there is no single "gay gene", there is overwhelming evidence of a biological basis for sexual orientation that is programmed into the brain before birth based on a combination of genetic and prenatal conditions, none of which are chosen by the fetus.
From: https://theconversation.com/stop-calling-it-a-choice-biological-factors-drive-homosexuality-122764