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The Biology of Same-Sex Attraction

June 16, 2014

Two Men Hugging

By Bill Dockery

The concept of two men or two women in a relationship has been known to raise a few eyebrows. Some religions deem homosexuality sinful and wrong. Conservative politicians clamor about negative impacts on traditional families and society. But for UT evolutionary biologist Sergey Gavrilets, homosexuality poses a completely different set of questions.

The persistence of same-sex coupling seemingly contradicts one of the fundamental notions of evolution, which states that biological characteristics not beneficial to the survival of the species should disappear over time. Theoretically, the trait should dead-end of its own accord because homosexual couples cannot produce biological offspring.

Gavrilets, an associate director of the National Institute for Mathematical and Biological Synthesis, and two colleagues recently addressed this evolutionary paradox in a paper that provides a giant step toward understanding the biological basis of homosexuality.

Published in the Quarterly Review of Biology, the study focused on epigenetics, a process that causes some genes to have temporary modifications known as epi-marks.

Like a film director instructing an actor to emphasize certain words, epi-marks are biochemical factors that determine how a gene’s embedded characteristics are expressed. Even though the script is the same, the director’s input can create dramatic differences.

In the context of homosexuality, the researchers surmise that the way epi-marks regulate a developing fetus’s sensitivity to testosterone during its initial growth could be the trigger.

The study claims that epi-marks may regulate the testosterone receptors critical to proper fetal development. In females, they can shield the fetus from high testosterone levels by decreasing the receptor’s sensitivity to the hormone. In males, they can stimulate the receptors when testosterone levels are low.

In most cases, epi-marks are “erased” as the fetus grows, but sometimes they persist and are passed on to the next generation. When this happens, the epi-marks active during a father’s development could cause his daughters to be overly sensitive to testosterone. Likewise, a mother’s epi-marks may manifest as a lack of testosterone sensitivity in her sons.

This means homosexual tendency in females is a product of the father, and that of males is a product of the mother. It may also explain the tendency of homosexuality to run in families.

To help support their hypothesis, the team developed a mathematical model to demonstrate how these epi-marks can spread throughout the population. Although the model suggests homosexuality can be inheritable, it does not require the existence of a “gay gene” that some have theorized about but failed to find. Without a recessive gene to be phased out over time, the evolutionary conundrum surrounding the endurance of homosexuality may be a moot point.

Gavrilets’s work, which is supported by the National Science Foundation, may provide new ammunition to the pop-culture drive to find a biological origin for same-sex attraction. The article sparked considerable coverage in both scientific and popular media. Publications and websites as diverse as Time, US News & World Report, Popular Science, Cosmos, and the New York Daily News reported the findings.

The major focus of Gavrilets’s ongoing research is on human origins and social evolution. “It is extremely exciting and also very rewarding to work in these areas, because it attracts the interests not only of scientists but also of the general public.” he says.

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