What role do sex chromosomes play in the identities of transgender people? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Biological gender and gender identity are two very distinct concepts. Biological gender or sex refers to the anatomy and physiology of a human body, whereas gender identity is influenced by a multitude of factors, most of which we don’t fully understand.

Biological sex is purely determined by the choice of sexual differentiation pathway, which is guided by genes on the sex chromosomes (though not exclusively, for example: WNT4 on chromosome 1). Testis-determining factor (TDF) or sex determining gene Y (SRY) located on the Y chromosome is one such gene. SRY is largely responsible for testis formation. It is not the only gene, and a variety of pathways and proteins are involved in this process of sex differentiation, some even located in the autosomal regions, but SRY is… special.

Figure 1: Cartoon image of the Y chromosome with the location of TDF or SRY gene[1].

The sex determining gene Y (SRY) was identified in the 1980s by Peter Goodfellow’s group [2]. Goodfellow’s group and others performed a series of experiments to demonstrate the role of the gene. In one such study, they looked at the genetic information of individuals who were anatomically female and had both XY chromosomes, and individuals who were anatomically male with XX chromosomes [3][4]. They found the SRY gene in an X chromosome in fifty cases of anatomically male with XX chromosomes. In one of the anatomically female case with XY chromosomes, they found a single nucleotide mutation in the SRY gene, that translated to a single amino acid change (from methionine to isoleucine), thus disrupting the testes development process. A single amino acid change from methionine to isoleucine in the SRY gene can cause an embryo with XY sex chromosomes to develop as a female. It is not difficult to imagine that such de novo mutations can also play a role in gender identity. In another study in 1991, they were able to transform female mice embryo to male (anatomically and behaviorally) by simply inserting one single SRY gene [5].

Gender identity is very poorly understood. But based on a number of studies from the popular case of David Reimer [who had gender reassignment surgery as a child and was brought up as a female after a botched circumcision and transitioned to a male at age 15], to multiple reports on genetically male (with XY chromosomes) and anatomically female who were brought up as female and later transitioned to male [6], it is safe to conclude that gender identity is less about behavioral, cultural, or social circumstances or upbringing and more biological.

While there are a number of genetic, biochemical studies [7][8][9][10] on gender identity, we don’t fully understand the involvement of specific genes. However, it is likely that genes [which in turn code for proteins, enzymes, hormones etc.], more than any other factor play a role in transgender identity. Can some of these genes reside on X or Y chromosome?—Perhaps. In my opinion, looking for biological variables as arbiters of gender identity is not a great idea for society.

Footnotes

[1] The School of Biomedical Sciences Wiki

[2] Peter Goodfellow

[3] Genetic evidence equating SRY and the testis-determining factor.

[4] "Genetic Evidence Equating SRY and the Testis-Determining Factor" (1990), by Phillippe Berta et al.

[5] Male development of chromosomally female mice transgenic for Sry

[6] https://www.ncbi.nlm.nih.gov/pmc...

[7] Genomic Characteristics of Gender Dysphoria Patients and Identification of Rare Mutations in RYR3 Gene

[8] A polymorphism of the CYP17 gene related to sex steroid metabolism is associated with female-to-male but not male-to-female transsexualism

[9] Pacific Center for Sex and Society

[10] Sex steroids and human behavior: prenatal androgen exposure and sex-typical play behavior in children.

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