The arrangement of a mother's genes could affect the sexual orientation of her son, according to a new study.
The finding, detailed in the February issue of the journal Human Genetics, adds fuel to the decade-long debate about whether so-called "gay genes" might exist.
The researchers examined a phenomenon called "X chromosome inactivation" in 97 mothers of gay sons and 103 mothers whose sons were not gay.
X and Y
Chromosomes are large thread-like molecules that contain an organism's genetic instructions. Humans have 23 chromosome pairs. The X chromosome is one of two sex chromosomes in mammals; the other is the Y chromosome. Females have two X chromosomes and no Y's, while males have one X and one Y.
Even though women have two X chromosomes, only one is functional because the other is inactivated through a process called "methylation."
"It gets wrapped up in a ball and is not used with the exception of a few genes," explained study leader Sven Bocklandt of the University of California, Los Angeles.
If one of the females' X chromosomes is not turned off, then there is too much genetic material, which can lead to a harmful overabundance of proteins. Down syndrome, for example, results from the presence of an extra copy of chromosome 21.
Normally, X chromosome inactivation occurs at random: half of the cells in a woman's body will have one X chromosome inactivated, while the other half inactivates the other chromosome.
However, when the researchers in the current study examined cells from the 42 mothers who had at least two gay sons, they found that about a quarter of the women in this group showed something different.
"Every single cell that we looked at in these women inactivated the same X chromosome," Bocklandt told LiveScience. "That's highly unusual."
In contrast, only 4 percent of mothers with no gay sons and 13 percent of those with just one gay son showed this type of extreme skewing.
Bocklandt thinks this suggest that a mother's X chromosomes partly influences whether her son is gay or not.
"We think that there are one or more genes on the X chromosome that have an effect on the sexual orientation of the sons of these mothers, as well as an effect on the cells we were looking at," Bocklandt said.
Other chromosomes implicated
Bocklandt was also involved in an earlier study that looked at the entire human genome of men who had two or more gay brothers. The researchers found identical stretches of DNA on three chromosomes—7, 8 and 10—that were shared by about 60 percent of the gay brothers in the study.
That study also found mothers to have an unusually large role in their son's sexual orientation: the region on chromosome 10 correlated with homosexuality only if it was inherited from the mother.
The results from these two studies suggest that there are multiple genetic factors involved in determining a person's sexual orientation and that it might vary depending on the person.
"We think that there are going to be some gay men who are X chromosome gay men and some who are chromosome 7 gay men or chromosome 10 gay men or some combination," Bocklandt said in a telephone interview.
Most researchers now think that there is no single gay gene that controls whether a person is homosexual or not. Rather, it's the influence of multiple genes, combined with environmental influences, which ultimately determine whether a person is gay.
A touchy subject
Research into the genetics of sexual orientation is controversial. Religious leaders who believe that sexual orientation is a choice argue that such research is an attempt to legitimize homosexuality; others worry that a detailed knowledge of the genetics underlying homosexuality will open the door to genetic engineering that prevents it.
But Bocklandt doesn't think these concerns should prevent scientists from asking the basic question of whether homosexuality has an underlying genetic component to it or not.
"I have no doubt that at some point we'll be able to manipulate all sorts of aspects of our personality and physical appearance," Bocklandt said. "I think if there's ever a time when we can make these changes for sexual orientation, then we will also be able to do it for intelligence or musical skills or certain physical characteristics—but whether or not these things are allowed to happen is something that society as a whole has to decide. It's not a scientific question."