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5.5: Genetics of Homosexuality, Transgender and Intersex

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    Scientists have wondered what biologically makes someone not binary in terms of sexual orientation or gender. The binary system is a culturally-created system that the U.S. and many other cultures uses to identify people. This traditionally means a person is male or female; a man or a woman; and attracted to the opposite sex. However, science is discovering that humans have way more variation than these two option, and that identity and orientation do not always match up. If your sex, gender, and orientation match what society expects, this is called being cisgendered. However, someone can be homosexual (attracted to the same sex), bisexual (attracted to either sex); they can be transgendered (biologically one sex but the gender of the opposite sex), intersex (having aspects and identities of both sexes), etc.

    There is evidence that a person’s identity and orientation are products of their genetics, hormones, environment and personal history. Here are some examples of this:

    • “Gay gene:” this is controversial, but if this is inherited, it would show up in identical twins more often than in fraternal twins.
      • A study by J. Michael Bailey showed homosexuality in 52% of identical twins and 22% in fraternal twins. This is more than twice the amount
      • A study by Dean Hamer showed twice the rate of homosexuality in identical vs. non-identical twins
      • There is a segment of the X chromosome (segment Xq28) that correlates to this and is passed down from mothers to sons
    • SRY gene: found on the Y chromosome; it determines the sex of an individual
      • A fetus is female until hormones turn this gene “on,” which then lets the body develop into a male form
      • When SRY genes are inserted into female mice they give birth to mice with XX chromosomes (so genetically female) but with male genitalia and male behaviors (including mating behaviors)
      • If this gene malfunctions or does not fully function, a person will not be “completely” male or female
    • DMRT 1 and FOXL2 genes: also help determine male sex and the development of sperm and testes
      • When these are removed from adult mice, the mice started growing female cells à the cells changed sex chromosomally
      • Males will develop ovaries
      • In an embryo, the sex is neutral until 6 weeks. Then, if the SRY gene turns on, the fetus will develop male. But, the DMRT1 gene keeps the fetus male. If this gene does not work, the FOXL2 gene will take over and make the body female
      • This is how people are intersex (they have biology of both sexes)
    • DHT and 5-alpha reductase: help a fetus develop into a female
      • To become female, since the fetus begins as female, it will just continue to develop naturally
      • For males, in addition to the genes we’ve discussed, it also needs the hormone testosterone and an enzyme called 5-alspha reductase
      • In a Dominican Republic village in the 1970s, researchers found “guevedoces,” which means “penis at 12.” Children who were female suddenly turned into males around puberty (aged 12). The reason is that they had the SRY gene and a Y chromosome, but they did not have enough of the DHT and 5-alpha reductase to make them develop as male until they received another surge of testosterone around puberty
      • Incredibly, about 95% of the children transitioned to being male with no problems. They fully embraced their new sex and gender

    What these cases show is that a person’s sex and gender identity are much more complicated than we once thought. While someone may choose to change his or her gender or sex, this is not so much a choice but rather them trying to align what their body and mind are telling them. Genetics, hormones, and enzymes play a huge role in shaping a person’s sex and gender. Additionally, we are finding many more options than the binary system mentioned above.

    Mukherjee, Siddhartha. The Gene: An Intimate History. (2016)

    Contributors and Attributions

    5.5: Genetics of Homosexuality, Transgender and Intersex is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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