We do not know for sure why some people are transgender. Many people think one or more genes of parents play a part in making their child transgender. People who study genes have found things that support this. The best guess in 2020: about 25% to 50% of being transgender comes from genes. Science does not know all the answers yet.
This page uses easy words. This helps young people read it. This also helps people who do not know many English words. The words in bold are hard. You need to know what they mean, or this will be hard to read. Start here if you do not know some of the words so far. You can use these links to looks up words you do not know:
Merriam-Webster’s Learner’s Dictionary
Cambridge Learner’s Dictionary
Why look for transgender genes?
Some people do not like transgender people. Many of us want to change that.
If science finds genes that make people transgender, it can show some good things:
- Being transgender is not a sin.
- Being transgender is not a crime.
- Being transgender is not a choice.
Many people who work in science think that being transgender is just a trait, like having brown eyes. It is not good or bad.
Some people think that being transgender is a bad trait. Some think it is a disease.
Other people worry about looking for transgender genes. They worry about how the science will be used.
If science finds genetic causes for being transgender, someone could make a genetic test to see if people had transgender genes. Some people think that a genetic test could be used for bad things. that violate laws or ethics. Some of the fears about a genetic test:
Testing babies for a transgender gene before they are born
- What if a pregnant person’s fetus could be tested for transgender genes? If the parents did not want a transgender child, they might end the pregnancy with an abortion. This already happens with other traits. People who think being transgender is a bad trait.
- About two out of three fetuses with trisomy 21 get aborted in the United States, because some people think trisomy 21 is a bad trait.
- About the same number of boys and girls are born each year, but some parents test their fetus. If the test says it is a girl, they abort it. They do this because they think being a girl is a bad trait. They think it is better to have a boy baby. One group thinks that 126 million girls were never born, just because they were girls.
- One person who wrote a bad book on transgender people wrote that if it is OK to abort a fetus for any reason, testing fetus and aborting the gay ones is OK.
- Many people believe that testing fetuses for transgender genes and terminating a pregnancy if the fetus is transgender would be unethical or immoral.
Testing people for a transgender gene before they get a job
- For instance, in 2017, President Donald Trump said that his administration would try to ban transgender people from serving in the US military. This would affect thousands of transgender military personnel currently serving, as well as anyone wishing to serve in the future. If trans people were banned and a gene test existed, that test could be used to enforce the transgender ban.
Testing people for a transgender gene to hurt them
- What if that kind of genetic test existed in one of the countries where being transgender is a crime? People could be tested and the transgender people could be imprisoned or killed.
Science: Good, bad, and fake
Some people claim that simply asking these ethical questions about scientific research is “anti-science.” That is not true. Science is simply a tool we use to understand the world. Tools can be helpful as well as dangerous if misused. You can use a hammer to build, and you can use it to scare or even kill someone. You can also make a mistake with a hammer and hurt yourself or others by accident. Mistakes in genetic research could hurt transgender people, even if the scientist did not mean to hurt anyone. That means the stakes are high.
Some controversial scientific research about trans people has been presented as a scientific debate, but it is really a debate about whether the research is good science, bad science, or fake science (called pseudoscience). Science works by trying to find out if the research was done according to the scientific method. That means having a hypothesis about something, then testing that hypothesis to see if the evidence matches the hypothesis. That conversation should include everyone, not just scientists. Sometimes people who are not researchers can see scientific mistakes the researchers missed.
Genetic research involving transgender people must avoid making any scientific mistakes, because even a tiny mistake could hurt a lot of transgender people. That is why it is important that everyone looks closely at genetic research for possible bias, especially the transgender people whose lives may be affected by the research.
Genetic and epigenetic basics
This page gets more and more technical. You need basic knowledge of human reproduction and genetics to read it. Here is a good overview on DNA:
There is probably not just one “transgender gene.” It is more likely that being transgender is a polygenetic trait, meaning that more than one gene can be involved.
Our genes are not the only thing that make us who we are. While heredity, or inheriting genes from our parents, is a big part of it, non-genetic factors can affect who we are, too. For instance, if pregnant people do something healthy like eat nutritious food, that can help a fetus, but if they do something unhealthy like smoke cigarettes, that can hurt a fetus. After we are born, how and where we grow up will affect all of us, too. Those factors after conception and before or after we are born are called the environment. Scientists are trying to figure out how much of who we are is caused by our heredity and how much is caused by our environment. Sometimes this proportion of heredity and environment is nicknamed “nature versus nurture.” That nickname is not quite right. It is not one or the other. Who we are is a blend of heredity and environment. Being transgender is probably a blend of nature and nurture.
Being transgender may also have an epigenetic aspect, where our environment affects our heredity. The study of epigenetics looks at biological mechanisms that will switch genes on and off.
This is all very hard to measure, which is why many scientists find it so interesting!
Transgender people who have a twin sibling
Transgender people who have a twin sibling are very interesting to genetic researchers, because sometimes the twins are both transgender. Gene scientists call this concordance. When twins are concordant for being transgender, it suggests that being transgender has genetic factors. It does not prove it definitively, but it shows a significant correlation.
Once a human egg gets fertilized, it is called a zygote. Twins happen for one of two reasons:
- One zygote splits in half, and the two halves become two babies with the same genes. These are called monozygotic twins (nicknamed “identical twins” and abbreviated MZ).
- Two zygotes are created at the same time. They are two ordinary siblings who happen to be made and born at the same time. These are called dizygotic twins (nicknamed “fraternal twins” and abbreviated DZ).
Some monozygotic twins both identify as transgender, like Jack and Jace Grafe:
However, for other monozygotic twins, only one identifies as transgender, even though they have the exact same genes, like Nicole Maines and her identical twin Jonas (shown below). Both were designated male at birth and raised as boys at first, but only Nicole identifies as transgender and made a gender transition. Scientists call this a discordance, the opposite of a concordant trait.
Gene scientists are interested in how often these two different kinds of monozygotic twins occur. They are also interested in comparing them to dizygotic twins.
I will give dizygotic sibling examples and discuss the concept of heterozygote advantage later.
Published case reports
Some published research uses outdated words. Those words are changed.
When healthcare professionals observe something interesting or unusual while they are working, they sometimes publish a case report in an academic journal. Over the years, case reports have documented dozens of transgender people with twin siblings:
- In 2012, a clinical team led by Australian Jacqueline K. Hewitt reported that 2 of the adolescents they treated were from dizygotic same-sex twin sibships, discordant for gender dysphoria.
Published twin research
Some published research uses outdated words. Those words are changed in some places.
Research into transgender people with twin siblings suggests that being transgender has genetic factors.
In 2011, a team led by Belgian researcher Gunter Heylens looked for all the published case reports about twins who were concordant or discordant for gender dysphoria. They found the following:
- among 23 pairs of monozygotic twins, 9 of them (39.1%) were concordant
- among 21 pairs of same-sex dizygotic twins, none of them (0%) were concordant
- among 7 pairs of opposite-sex twins, all (100%) were discordant for GID.
Sex researcher Milton Diamond wanted to see if he could find patterns of transgender traits by comparing monozygotic twins to dizygotic twins. This is called a study of concordance.
Diamond found 61 pairs of monozygotic twins where at least one had made a gender transition. Then he compared them to 38 pairs of dizygotic twins where at least one had made a gender transition. One interesting finding was that 3 of the 99 sets of twins were concordant for transition even though they were separated at birth. In other words, the twins grew up in different environments and did not know each other. Imagine having a twin you never met, and both of you transitioned after living in totally separate homes and never knowing about each other. Those 3 sets of twins suggest that there is a genetically heritable part of why they are transgender.
Diamond also found out that of the 38 pairs of dizygotic twins, only one pair was concordant, where both dizygotic twins had transitioned. Then he found that of the 61 pairs of monozygotic twins, 21 of those pairs were concordant, where both monozygotic twins had transitioned. Diamond’s study found that monozygotic twins were 10 times more likely than dizygotic twins to be concordant for both making a gender transition. Even though it would be ideal to have a lot more transgender twins to study, that significant difference strongly suggests that there is a genetically heritable part of why they are transgender.
Diamond’s findings are very interesting, but it is not enough evidence to prove that being transgender is genetic.
Outside of academia, most people have a very oversimplified understanding of genetics, especially when it comes to sex, gender, and sexuality. As children, they learned “boys are XY and girls are XX” and that sums everything up. This kind of belief is called genetic essentialism.
Related gene research
In 2018, Tinca Polderman led a team of researchers who wrote: “Based on the data reviewed, we hypothesize that gender identity is a multifactorial complex trait with a heritable polygenic component.”
In 2019, J. Graham Theisen led a team that that summarized earlier studies: “In adolescents, these studies reported heritability estimates between 38–47% in [trans men] and 25–43% in [trans women], while in adults, estimates ranged between 11–44% and 28–47% respectively.” Their own study found: “After filtering, 441 variants in 421 genes remained for further consideration, including 21 nonsense, 28 frameshift, 13 splice-region, and 225 missense variants. Of these, 21 variants in 19 genes were found to have associations with previously described estrogen receptor activated pathways of sexually dimorphic brain development.”
In addition to researching possible genetic roles in gender identity and expression, scientists are also looking into sexual orientation and differences of sex development, because those traits may be connected to being transgender.
Garg and Marwaha note “it is thought to originate from a complex biopsychosocial link.” I’ll explain those complicated interactions at a later date, along with gene polymorphism of the gene that affects androgen receptors.
Theisen JG, Sundaram V, Filchak MS, Chorich MP, Sullivan ME, Knight J, Kim H-G, Layman LC (2019). The Use of Whole Exome Sequencing in a Cohort of Transgender Individuals to Identify Rare Genetic Variants. Sci Rep. 2019 Dec 27;9(1):20099. doi: 10.1038/s41598-019-53500-y. https://doi.org/10.1038/s41598-019-53500-y
Polderman TJC, Kreukels BPC, Irwig MS, Beach L, Chan YM, Derks EM, Esteva I, Ehrenfeld J, Heijer MD, Posthuma D, Raynor L, Tishelman A, Davis LK (2018). The Biological Contributions to Gender Identity and Gender Diversity: Bringing Data to the Table. Behav Genet. 2018 03; 48(2):95-108. https://doi.org/10.1007/s10519-018-9889-z
Hewitt JK, Paul C, Kasiannan P, Grover SR, Newman LK, Garry L Warne GL (2012). Hormone treatment of gender identity disorder in a cohort of children and adolescents. Medical Journal of Australia 2012; 196 (9): 578-581. https://doi.org/10.5694/mja12.10222
Dewing P, Shi T, Horvath S, Vilain E (2003). Sexually dimorphic gene expression in mouse brain precedes gonadal differentiation. Molecular Brain Research 118:82-90. https://doi.org/10.1016/S0169-328X(03)00339-5
Green R (2000). Family co-occurrence of gender dysphoria: ten sibling or parent-child pairs. Archives of Sexual Behavior. 29:499-507. https://doi.org/10.1023/A:1001947920872
Heylens G, De Cuypere G, Zucker KJ, Schelfaut C, Elaut E, Vanden Bossche H, De Baere E, and T’Sjoen G (2012). Gender identity disorder in twins: A review of the case report literature. J Sex Med 2012;9:751–757. https://doi.org/10.1111/j.1743-6109.2011.02567.x
Diamond M (2013). Transsexuality Among Twins: Identity Concordance, Transition, Rearing, and Orientation. International Journal of Transgenderism Volume 14, 2013 – Issue 1, pp. 24-38. https://doi.org/10.1080/15532739.2013.750222
Diamond M, Hawk ST (2013). Transsexualism among twins has a high concordance for GID among monozygotic twins and a strong but lesser concordance among dizygotic twins with the effect more noticeable among males than females. Paper presented at the Harry Benjamin International Gender Dysphoria Association Symposium, Ghent, Belgium, September 10–13, 2003.
Fernández R, Cortés-Cortés J, Esteva I, Gómez-Gil E, Almaraz MC, Lema E, Rumbo T, Haro-Mora JJ, Roda E, Guillamón A, Pásaro E. (2015). The CYP17 MspA1 polymorphism and the gender dysphoria. J Sex Med. 2015;12(6):1329–1333. https://doi.org/10.1111/jsm.12895
Garg G, Marwaha R (2018). Gender Dysphoria (Sexual Identity Disorders). StatPearls. https://www.ncbi.nlm.nih.gov/books/NBK532313/
Bentz EK, Hefler LA, Kaufmann U, Huber JC, Kolbus A, Tempfer CB. A polymorphism of the CYP17 gene related to sex steroid metabolism is associated with female-to-male but not male-to-female transsexualism. Fertil Steril . 2008;90(1):56–59.
Strang JF, Kenworthy L, Dominska A, Sokoloff J, Kenealy LE, Berl, Walsh K, Menvielle E, Slesaransky-Poe G, Kim KE, Luong-Tran C, Meagher H, Wallace GL. Increased gender variance in autism spectrum disorders and attention deficit hyperactivity disorder. Arch Sex Behav . 2014;43(8):1525–1533.
Ujike H, Otani K, Nakatsuka M, Ishii K, Sasaki A, Oishi T, Sato T, Okahisa Y, Matsumoto Y, Namba Y, Kimata Y, Kuroda S. Association study of gender identity disorder and sex hormone-related genes. Prog Neuropsychopharmacol Biol Psychiatry . 2009;33(7):1241–1244.
Henningsson S, Westberg L, Nilsson S, Lundström B, Ekselius L, Bodlund O, Lindström E, Hellstrand M, Rosmond R, Eriksson E, LandénM. Sex steroid-related genes and male-to-female transsexualism. Psychoneuroendocrinology . 2005;30(7):657–664.
Lombardo F, Toselli L, Grassetti D, Paoli D, Masciandaro P, Valentini F, Lenzi A, Gandini L. Hormone and genetic study in male to female transsexual patients. J Endocrinol Invest . 2013;36(8):550–557.
Fernández R, Esteva I, Gómez-Gil E, Rumbo T, Almaraz MC, Roda E, Haro-Mora JJ, Guillamón A, Pásaro E. Association study of ERβ, AR, and CYP19A1 genes and MtF transsexualism. J Sex Med . 2014;11(12):2986–2994.
Fernández R, Esteva I, Gómez-Gil E, Rumbo T, Almaraz MC, Roda E, Haro-Mora JJ, Guillamón A, Pásaro E. The (CA)n polymorphism of ERβ gene is associated with FtM transsexualism. J Sex Med . 2014;11(3):720–728.
Cortés-Cortés J, Fernández R, Teijeiro N, Gómez-Gil E, Esteva I, Almaraz MC, Guillamón A, Pásaro E. Genotypes and haplotypes of the estrogen receptor α gene (ESR1) are associated with female-to-male gender dysphoria. J Sex Med . 2017;14(3):464–472.
Hare L, Bernard P, Sánchez FJ, Baird PN, Vilain E, Kennedy T, HarleyVR. Androgen receptor repeat length polymorphism associated with male-to-female transsexualism. Biol Psychiatry . 2009;65(1):93–96.
Diamond M. Transsexuality among twins: identity concordance, transition, rearing, and orientation (2013). Int J Transgenderism . 2013;14(1):24–38.
Furtado PS, Moraes F, Lago R, Barros LO, Toralles MB, Barroso U Jr. Gender dysphoria associated with disorders of sex development (2o12). Nat Rev Urol . 2012;9(11):620–627.
Mattila AK, Fagerholm R, Santtila P, Miettinen PJ, Taskinen S. Gender identity and gender role orientation in female assigned patients with disorders of sex development (2012). J Urol . 2012;188(5):1930–1934.
Garg G, Marwaha R (2018). Gender Dysphoria (Sexual Identity Disorders). StatPearls https://www.ncbi.nlm.nih.gov/books/NBK532313/
Fisher AD, Ristori J, Fanni E, Castellini G, Forti G, Maggi M. Gender identity, gender assignment and reassignment in individuals with disorders of sex development: a major of dilemma (2016). J Endocrinol Invest . 2016;39(11):1207–1224.
Mendonca BB. Gender assignment in patients with disorder of sex development (2014). Curr Opin Endocrinol Diabetes Obes . 2014;21(6):511–514.
de Vries A, Noens I, Cohen-Kettenis P. Autism spectrum disorders in gender dysphoric children and adolescents. J Autism Dev Disord 2010; 40: 930-936.
Yang JH, Baskin LS, DiSandro M (2010). Gender identity in disorders of sex development: review article. Urology . 2010;75(1):153–159.
Sandberg DE, Gardner M, Cohen-Kettenis PT (2012). Psychological aspects of the treatment of patients with disorders of sex development. Semin Reprod Med . 2012;30(5):443–452.
Meyer-Bahlburg HF, Baratz Dalke K, Berenbaum SA, Cohen-Kettenis PT, Hines M, Schober JM (2005). Gender assignment, reassignment and outcome in disorders of sex development: update of the 2005 Consensus Conference. Hormone Research in Paediatrics. 2016;85(2):112–118. doi: 10.1159/000442386
Jürgensen M, Kleinemeier E, Lux A, Steensma TD, Cohen-Kettenis PT, Hiort O, Thyen U, Köhler B; DSD Network Working Group (2013). Psychosexual development in adolescents and adults with disorders of sex development–results from the German Clinical Evaluation Study. J Sex Med . 2013;10(11):2703–2714. doi:10.1111/j.1743-6109.2012.02751.x
Foreman M, Hare L, York K, Balakrishnan K, Sánchez FJ, Harte F, Erasmus J, Vilain E, Harley VR (2018). Genetic link between gender dysphoria and sex hormone signaling. The Journal of Clinical Endocrinology & Metabolism, Volume 104, Issue 2, February 2019, Pages 390–396, doi:10.1210/jc.2018-01105
Choong CS, Wilson EM. Trinucleotide repeats in the human androgen receptor: a molecular basis for disease. Journal of Molecular Endocrinology. 1998;21(3):235–257. doi:10.1677/jme.0.0210235
Jones Z (2018). Pervasive misrepresentation of twin evidence for a genetic component of gender dysphoria. Gender Analysis. https://genderanalysis.net/2018/03/pervasive-misrepresentation-of-twin-evidence-for-a-genetic-component-of-gender-dysphoria/