The idea that biological sex is a binary aspect is very widespread. Two possible combinations of sex chromosomes, XX or XY , providing two distinct gonads, ovaries or testes, and developing two possible genital systems: one consisting of the clitoris, vulva, vagina, uterus, and associated female reproductive system; and another formed by the penis, scrotum, prostate, and the male reproductive system. At the time of maturation, the rest of the secondary sexual characteristics develop. Result: women and men .
This oversimplified scheme of human sexuality ignores many factors. Among them, the confusion that “woman” and “man”, gender identities, are purely biological categories, when in fact they are part of a spectrum formed by bio-psycho-social aspects —that is, gender is determined by a combination of biological, psychological, and social factors.
But, gender identities aside, and reducing ourselves only to sex, the presented scheme continues to ignore many realities. Among others, the fact that the presence of testicles or ovaries is not defined by chromosomes, but by a specific gene, the SRY gene. It is the gonads that, through hormones, develop the rest of the primary and secondary sexual characteristics. The SRY gene is normally present on the short arm of the Y chromosome, but, exceptionally, it can be translocated to the X chromosome.
That is, there is a possibility that a person with XX chromosomes has a translocated SRY gene and develops testes , reproductive system and secondary male characteristics. Or conversely, someone with XY chromosomes may lack the SRY gene—or have it silenced by some mutation —and develop ovaries, a reproductive system, and secondary female traits as a result .
And there are even more complex scenarios. People with a different sex chromosome set —and with different results—: with a single X chromosome, with three chromosomes —XXY, XYY—… and even cases of chimerism .
Chimerism is a phenomenon whereby two genetically distinct embryos fuse early in embryonic development to form a single individual. If each embryo carries a different set of chromosomes, the person may have some of their cells with XX chromosomes and others with XY.
In such cases, it may happen that they develop a testicle on one side and an ovary on the other, or even ovotestis , a gonad with testicular and ovarian tissues simultaneously. If this occurs, the resulting genitalia are usually ambiguous, intermediate between male and female. People with these intermediate characteristics are called intersex . Its mere existence demonstrates that sex in human beings is far from being a simple ‘black and white’ binary trait.
Nature is so diverse , and the sexual determination system in humans so complex, that changes in some of its variables yield surprising results.
Androgens , like testosterone , are hormones produced by both the ovaries and the testicles, except that the latter produce it in much greater quantity. In both cases, this hormone is responsible for the production of body hair -especially axillary, pubic and facial hair-, and the increase in libido. When elevated levels of testosterone are present, other secondary traits develop such as larger vocal cords – deepest voice -, greater muscle development, growth of the Adam’s apple, widening of the jaw… and, of course, in maximum concentrations, the development of male genitalia.
Testosterone is metabolized, thanks to the aromatase enzyme , into estradiol , one of the hormones responsible for the development of secondary female characters, both primary, the development of female genitalia, and secondary, such as widening of the hips and growth of the breasts during puberty. The ovaries also produce estradiol , so people with these gonads have higher concentrations than those with testes.
Taking all of this into account, we know that a person who has XY chromosomes and a functional SRY gene develops testicles that produce androgens. But for these hormones to fulfill their functions, the cells must have specific receptors , whose synthesis is regulated by another gene, the AR .
If the AR gene is missing or non-functional, the androgen receptor is not synthesized and the cells of that person’s body cannot recognize and assimilate testosterone. This insensitivity to testosterone in people with XY chromosomes is called Morris syndrome , and it prevents them from developing male sexual characteristics. Part of this unassimilated testosterone is metabolized into estradiol, which performs its functions by developing the female primary and secondary sexual characteristics.
The result: people with XY chromosomes, with a pair of undescended testicles housed where ovaries would usually be; without a uterus, with a blind vagina, clitoris and female external sexual apparatus, and also female secondary features.
The insensitivity to testosterone prevents them from developing angular forms, exaggerating secondary feminine features, making them reach adulthood without developing facial, axillary, or pubic hair. They are sterile and do not menstruate.
Morris syndrome is not usually detected at birth, people born with this condition are normally assigned the female gender . Typically, they maintain their gender identity into adulthood and identify as female . The absence of androgen receptors also prevents testosterone from having an effect on the brain; relevant, although not unique, aspect of gender identity.
Although many claim that biological sex is a binary aspect, nature is enormously diverse, it has its own rules of the game, generally much more complex than the contrived categories that we humans invent to try to explain them. The truth is that both biological sex and gender identity , which is bio-psycho-social, are complex traits , which depend on multiple variables in several dimensions, which form spectra, with distributions that are sometimes clear, but with not a few cases that go beyond the artificial limits that we comfortably try to define.
If anthropomorphization is admitted as a metaphorical resource: nature likes diversity .
References:
Borrego, LJA et al. 2012. Morris Syndrome. Cuban Journal of Obstetrics and Gynecology , 38(3).
D. Dell’Edera et al. 2010. Androgen insensitivity syndrome (or Morris syndrome) and other associated pathologies. European Review for Medical and Pharmacological Sciences , 14, 947-957.
Martinez-Glez, V. (Ed.). 2013. Androgen insensitivity syndrome . Working Group on Cancer in Polymorphic Genetic Syndromes.
Nedia, M. et al. 2009. Morris syndrome. The Gynecologist , 1(2), 19-21. Villafañe, VE et al. 2011. True hermaphroditism. A 46XY/46XX case. Pathology – Latin American Journal , 49(2), 138-140.
