November 09, 2004
X Chromosome Methylation Pattern Causing Some Male Homosexuality?

Normally in each cell of a female body one of the two X chromosomes has been deactivated by the placement of methyl groups (a carbon and 3 hydrogens) along the backbone of the DNA double helix. That deactivation is random. Suppose there are two X chromosomes in a woman called A and B where the A chromosome came from her mother and the B chromosome came from her father. In approximately half of the cells the A's will be deactivated and in the other half of the cells the B's will be deactivated. Well, in women who have gay sons the deactivation of the X chromosomes is skewed with one of the X's more often deactivated than the other.

But when Sven Bocklandt of the University of California, Los Angeles, compared blood and saliva samples from 97 mothers of gay men with samples from 103 mothers without gay children he found this process was extremely skewed in the mothers with gay sons, with one X chromosome being far more likely to be inactivated than the other. Only 4% of the mothers without gay sons showed this skewing, compared with 14% of mothers with at least one gay son. Among mothers with two or more gay sons, the figure was 23%.

This result suggests that eventually any woman will be able to have a genetic test done what will predict the odds that one of her sons will be born homosexual.

One can easily imagine all sorts of responses to this. It is safe to say that most women and most men do not want to have homosexual children. The degree of that preference varies and some people do not have a strong preference against their own children being homosexual. There is no doubt even a minority of people who would prefer their children to be homosexual.

So will women who are at greater risk of having homosexual sons become more reticent to have children? Or will they use reproductive technologies such as MicroSort to tip their odds toward having a daughter instead?

One question unanswered by this study is whether the skewing of X chromosome shut-down also increases the odds of lesbianism. The mechanism for producing lesbian female children may be (and I'm guessing is) entirely unrelated to the mechanism for producing homosexual sons. But a follow-up study could easily answer this question by repeating the same analysis with the mothers of lesbian daughters.

Of course, understanding of a genetic mechanism for the development of homosexuality will eventually lead to the development of treatments to alternatively prevent or cause a homosexual outcome. One question I do not have a guess about is whether control over sexual orientation of offspring will produce more or fewer homosexuals. You might argue that since most people either prefer heterosexual children or have no preference (or are at least so politically correct that they won't publically admit to a preference) it is hard to see how control over sexual orientation could increase the number of homosexuals. But there is one big point to consider: It takes only a small fraction of the population to choose having a homosexual child to increase the number that are born. Suppose that only 3% of male births are homosexual. It would take only 1 in 20 women (or gay men who hire a surrogate woman to carry a baby) deciding they'd rather have homosexual son to increase the number born.

My guess is that there is a greater chance that control of reproduction could increase the number of lesbians. After all, it is women who carry the babies, not men. Will most lesbian women want to have heterosexual or homosexual daughters? Anyone have a reason to think they know the answer?

This question of preferences for offspring sexual orientation is part of what I consider to be a huge coming issue: when people can control more characteristics of their offspring what choices will they make? Some choices are not hard to guess. People will want their kids to be smarter and better looking. But what will be the chosen frequency of, say, blondness or introversion versus extroversion? Or height? Tall overall is my guess. But how much taller? Will the distribution of heights become narrower since people won't want to burden their kids with excessive height? (that is my guess). Will personality choices for offspring become problematic for society as a whole? See my posts Altruistic Punishment And Genetic Engineering Of The Mind and Brain Rewards For Carrying Out Altruistic Punishment for more on that question.

Share |      Randall Parker, 2004 November 09 04:26 PM  Brain Genetics

Eric said at November 10, 2004 8:07 PM:

Any idea when this will be published?

Peter said at November 11, 2004 2:05 PM:

>Will most lesbian women want to have heterosexual or homosexual daughters?
From the ones I've known well enough to talk about it, they have all expressed preference for gay sperm donors. From the literature I've read, the majority seem to prefer that their children also be gay, but acknowledge that their kids appear to have a higher percentage of heterosexuals than heterosexual couples do.

Mr. George said at December 1, 2004 10:59 AM:

Yeah, I agree with Podcaster on that one. You can't forget about the miniature hippos on those cold days. They are likely to freeze up causing a huge atomical explosion with the power of a stick of firewood.

Nyarlathotep said at February 24, 2006 4:16 PM:

Okay, couple questions I can't find answers for on other sites either:

Of the X chromosomes in the woman who's the mother of the gay children, it says the deactivation is skewed towards one of her parents' X's being deactivated more often. Which one, the one she got from her mother or the one from her father?

Were these women's parents also involved in the study so they can determine which X came from where? If not, how could you tell which X came from which parent?

Stephen Jones said at December 11, 2010 12:17 AM:


It does not matter where the X comes from. The X is passed on, intact, from mother to daughter, and from mother to son. In a son the X with the methylation pattern will usually result in homosexuality. In her daughters, if this X is expressed, daughters may be more masculine. If it is not expressed, and the other X is favoured, the methylation pattern will be silent - she will be a silent carrier in other words.

In women with two X chromosomes exhibiting this methylation pattern, feminine sexual traits may be largely suppressed - producing a lesbian woman, and perhaps even a butch lesbian woman.

Gay men who have children will pass on the methylated X to their daughters, but they pass on their Y chromosome to their sons. If the mother does not have the methylated X, the homosexual man's sons will be heterosexual.

The X chromosomes stay intact from generation to generation - the homosexuality that results can skip generations.

All the code for sexual attraction needs to be on the X and Y. Most likely it is all on the X, with the Y working as a kind of master "enabler" in men controlling the expression of genes on the X and on other genes inherited from either parent.

For women, with two ordinary X chromosomes, the code for attraction to women is muted and the code for attraction to men is expressed. It is not an exact science - and many women retain an attraction to other women, just as many heterosexual men retain a physiological attraction to men.

In fact, it is likely that when there is no social pressure to be heterosexual, many more men and women would allow their physiological responses to other men to be part of their normal behaviour - bisexuality may be a more "normal" state for men and women with "normal" X chromosomes. Certainly, we know that many men refrain from pursuing homosexual encounters because of social pressure.

In homosexual men it is not only the code for sexual physiological attraction to men which is left turned on. There is evidence to suggest that other specialist attributes of the female are also left turned on, when they would normally be turned off by the Y. Gay men are often considered to be more sensitive; more collegial; more psychic; more creative; more artistic etc than heterosexual men.

It is my belief that, over time, valuable characteristics migrate from other chromosomes to the X and the Y. I don't know what mechanism makes this possible, but it makes sense for an organism to ensure that its most valuable traits are passed on to all its children. If true, this means that homosexual men enjoy all the best characteristics of their fathers, and also the best characteristics of their mothers. This would confer an advantage, not only on the homosexual son, but also on the community in which the homosexual lives.

It also makes sense that females will usually ensure that their best characteristics are generally only expressed in their daughters. This might explain the often poor relationship that homosexual men have with their mothers: it may be an instinctive reaction to the expression of their best genes in the "enemy" sex. Females use a lot of devices, acquired over many generations, to balance the power of men. Homosexual men do not respond physiologically to the sexual wiles of women, and will often observe the devices (sexual trickery) deployed by the woman. That might annoy their mothers too! Some homosexual men may also express the code for these devices and recognise it because they use this code also.

If you observe homosexual men you may note that there are different "types". Some are feminine - some very feminine - while others are largely indistinguishable from heterosexual men. Perhaps the feminine men have mothers who have inherited an X chromosome where the most useful characteristics include their femininity - perhaps selected for by a culture in which women have to compete sexually for men. In other cultures, or even in social groups within a culture, the women do not have to compete and do not need to develop such a pronounced femininity. In a family, where marriages are usually arranged, the women are often stronger and less overtly feminine in their behaviour. Homosexual men with mothers from these families will express these other attributes instead.

All in all, a fascinating subject...

mick said at March 29, 2011 11:08 AM:

@Stephen Jones,

Your analyses demonstrates no knowledge of evolutionary biology.

The results of the previous Bocklandt study were intriguing. The results of his current epigenetics study are either not yet in or not yet written up.

Genes turn on and off for a variety of reasons and inactivation of a chromosome can occur as a result of different circumstancs. For instance, methylation can be a response to an outside agent such as a pathogen or even an autoimmune response to one.

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