The role of genetics in the cause of homosexuality or transsexuality is a complex debate and is far beyond my capability to cover responsibly. One of the best sources I’ve seen on it is Affirming God’s Image: Addressing the Transgender Question with Science and Scripture by J. Alan Branch.
I will offer a few quotes here to give you a feel for his answers. But if you want to explore this further, this book would be an excellent place to start.
As with any “cause” question, this evidence is unlikely to change the mind of a person who uses the argument. These
Is there any research that proves a genetic cause?
“A careful review of research regarding genetics and transgenderism reveals that transgenderism is not an inherited trait like skin color. Instead, current research demonstrates that while genetic variations may serve as contributing factors to the complex identity known as transgenderism, the evidence that transgenderism is caused by genetics is quite weak. This distinction between a contributing factor and causation is essential:
“A contributing factor increases the likelihood that something may happen; causation means that something produces a definite effect. As we will see, to date, no studies have found specific genes that cause transgenderism. Pg. 56
The main theories Branch discusses are as follows:
The Prenatal Hormone Theory of Transgenderism
“…the most common suggestion is the prenatal hormone theory, which says transgenderism arises in the womb from a discrepancy between sexual differentiation of the genitals and sexual differentiation of the brain. In the womb, the genitals develop first and the brain experiences sexual differentiation later. Thus, this theory suggests that, early on in prenatal development, the genitalia of someone who is “transgender” develops in accordance with his or her genetic sex, but then an aberration occurs and later the brain develops in a manner consistent with the opposite sex, resulting in someone who claims to have a “male” body with a “female” brain or a “female” body with a “male” brain.” (Affirming God’s Image: Addressing the Transgender Question with Science and Scripture by J. Alan Branch.” Pg. 56)
Branch relates this to another theory which is:
Transgenderism And Genes Related To Androgen And Estrogen Receptors
“If the prenatal hormone theory is true, some argue that the reason the brain develops inconsistently with the genitals is related to the genes for androgen receptors and estrogen receptors. Among the many aspects of the amazing human body God has designed, androgen and estrogen receptors are special proteins that have a tremendous influence over our sex. In general, receptor proteins serve as the “eyes and ears” of cells, receiving messages from substances in the bloodstream and then telling the cells what to do…
“Because androgen and estrogen receptors are so important in sexual development, some researchers have tried to discover if transgender people have unique genes related to this process that might cause them to be transgender. Because testosterone (an androgen) and estrogen are essential to the normal development of males and females, it makes sense to examine genes that affect the way the body processes these hormones if one thinks transgenderism has a genetic cause.” (Affirming God’s Image: Addressing the Transgender Question with Science and Scripture by J. Alan Branch. Pg. 57-8)
However, he concludes:
“Research on genes related to androgen receptors and estrogen receptors in MtF and FtM transsexuals had yielded widely divergent results, though some findings are intriguing. The results are contradictory; nothing has been proved conclusively concerning these particular genes and transgender identity. (Affirming God’s Image: Addressing the Transgender Question with Science and Scripture by J. Alan Branch. Pg. 59)
The bottom line: there is no clear evidence of a genetic cause for homosexual or transgender identity.
Branch continues to discuss the specific research on particular genes and brain structure. Again he concludes there is no clear evidence that genetics are a cause of homosexuality:
“THE CAUSE OF TRANSGENDERISM IS UNKNOWN: After reviewing data concerning the causes of transgenderism, we can conclude that the causes of transgenderism are unknown, but hormonal, genetic, brain organization, and environmental variables are all likely to be contributing factors. But this is far, far different from saying transgenderism is caused by any innate characteristic or that transgender people are “born this way.” While research certainly has more to learn regarding the brains of transgender people, current data do not prove a third brain type exists.” (Affirming God’s Image: Addressing the Transgender Question with Science and Scripture by J. Alan Branch. Pg. 78)