Genetic Basis for Sexual Orientation
The mechanism of gene selection
Physical, physiological and behavioral traits are a result of a complex interaction between genetic and environmental factors.
Forming the basis for these biological characteristics are the genes. The genes which make up the human genome are contained in 23 pairs of chromosomes. One set of these chromosomes comes from the male gamete while the other comes from the female gamete.
During fertilization the genes from the male and female gametes come together to form complimentary pairs. Except for a few gamete specific genes, all other genes are left in an activated or unmethylated state after fertilization. This allows the embryonic stem cells to have the poly-potential to become any of the cell types required to form all the structures of the body. This set of potential characteristics is the person's genotype.
However, the presence of two genes, each attempting to specify the same characteristic, is not only unnecessary but may also be harmful. Therefore, for most pairs of genes, one is left active while the other is deactivated through the process of methylation. The set of genes which are left active determines the person's actual characteristics or phenotype. If the father has blue eyes and the mother has brown eyes, each of their children are likely to have either blue eyes or brown eyes, but never blue-brown eyes. This process of gene selection takes place for the thousands of gene pairs. Once in place, this pattern of methylation is preserved during cell division so that the initial set of characteristics is replicated in every cell of the body.
A person's sex is specified by an entire chromosome. The presence of the male Y chromosome in the XY combination results in a male offspring while the repetition of the female X chromosome in the XX combination produces a female offspring.
In addition to characteristics which are determined by a single gene or an entire chromosome, many are determined by a related set of genes, known as a gene complex (gc). Control areas preceding these gc's are such that the entire set of genes can be switched off or left on as a whole.
Since autonomic nervous system responses like the "fight or flight" reaction are preprogrammed in our genes, there must also be a genetic basis for the similar physiological reactions which occur when one person is sexually attracted to another. Since most people are not attracted to members of both sexes, there must be one set of genes which would lead a person to be attracted to males (AtoM) and another set which would lead a person to be attracted to females (AtoF).
The hypothalamus and other portions of the limbic system play an integral role in emotional responses including those related to sexual attractedness. Studies of the fragile X syndrome show that the X chromosome is involved in brain development and function in this portion of the limbic system. The AtoM gc and AtoF gc may be located on the X chromosome as well.
Like almost all genes in the gametes, those in the X and Y chromosomes are all initially found in an activated state. However, the Y chromosome's role is limited, after all females don't even have one, and males do fine with only one X chromosome, so the second X chromosome in females is all but superfluous. So at some time during embryonic development, the Y chromosome in males and one of the two X chromosomes in females are inactivated with the exception of a few complimentary areas.
Yet these sex chromosomes must perform a role in determining primary and secondary sexual characteristics. After all, men and women are different. They must also play a role in determining sexual orientation. When the Y and X chromosomes from the male and female gametes form a pair during fertilization, the presence of the Y chromosome may provide the signals which would usually result in the AtoM gc on the X chromosome being turned off while the AtoF gc is left on. This would result in a man who is attracted to females. When the male gamete provides a second X chromosome instead, the signals would usually result in the AtoF gc being turned off while the AtoM gc is left on. This would result in a woman who is attracted to males.
However, if the pairing of the Y and X chromosomes is such that the usual Y signals are weak or poorly responded to by the X chromosome receptors, this could result in the AtoF gc being turned off while the AtoM gc is left on. This would result in a man who is attracted to males. Similarly, the pairing of two X chromosomes may provide similarly ambiguous signals and the AtoM gc may be turned off while the AtoF gc is left on. This would result in a woman who is attracted to females. In either the XY or XX combination, the signaling could be such that both the AtoM and the AtoF gc's are left on. This would result in a man or woman who is attracted to both males and females.
Since all the necessary biological functions are provided by a single X chromosome, the second X chromosome in females is all but completely turned off. However, this may not always be the case. In certain rare instances the AtoM gc on the usually inactive X chromosome may be active. With both AtoM gc's active, the result would be a woman who is doubly attracted to males, a condition know as nymphomania. Since the attractedness gc's are located on the X chromosome and men have only one X chromosome, there is no corresponding condition among men.
Another possibility exists. We may have a man with both the AtoF and the AtoM gc's turned off. This person may have dated some in high school but he soon loses interest in this ritual. Content in other areas of his life, he becomes the perennial bachelor who is simply not inclined toward forming an intimate relationship with anyone.
Last we may have a woman with both the AtoF and the AtoM gc's turned off. She may still have a strong maternal instinct and may therefore marry in order to have children, but sexual relations beyond that are considered more of a duty than a pleasure. Or she may be the maiden aunt that is somewhere in every family.
These asexual conditions are rarely studied since a lack of sexual interest is unlikely to result in the type of problems which would lead one to seek therapy. Yet these can cause problems when it comes to public policy regarding such things as sexuality education in our schools. When one mentions the powerful urges that occur as girls and boys mature, they haven't the foggiest notion what one is talking about. Having never experienced being sexually attracted to anyone themselves, they cannot understand why our youngsters can't "just say no" to sexual activity as easily as they did when they were their age.
If sexual orientation is determined by the post conception activation of certain gene complexes on the X chromosome, then the following would apply:
1. Everyone has an X chromosome and all X chromosomes carry both the AtoM and AtoF gc's. There is no unique "homosexuality gene."
2. Since current testing can detect the presence of genes, not which ones are turned off or left on, there is no genetic test for sexual orientation. A physiological test is used in clinical studies.
3. Since these gene sets are either on or off, there is no spectrum of limbic system reactions going from mostly heterosexual to mostly homosexual. A person is all one or the other (or both or neither.) Behavior is voluntary and is an entirely different matter as described here.
4. A person cannot change his or her sexual orientation. No amount of intellectual introspection or prayerful reflection can change the sex specific imprinting that leads to the sexual arousal response in the limbic system. Such "Magical Thinking" is described here.
5. Being around someone who is gay will not encourage someone else to become gay. If one is not born gay, one will not become gay later.
6. A person's sexual orientation is nobody's "fault." It is not the person's fault or his or her parents' fault or society's fault. Homosexuality is not a "sin" since it is not freely chosen. Excerpts from the American Psychological Association's Answers to Your Questions About Sexual Orientation and Homosexuality are discussed here.
7. Since the same AtoM gc is active in gay men as in heterosexual women, the limbic system responses will be similar and can be managed as easily by gays as they are by females.
This material has been revised since its original 1995 publication. Any suggestions for further revision would be welcome.
Update See my recent post Gay Limbic Response to Pheromones which relates how the sexual arousal response in the hypothalamus to odors is the same for homosexual men as it is for women.