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Sexual Differentiation and Gender Roles

Cayla McNally's picture

As we evolve from zygotes to fully-functioning adults, we are influenced by a myriad of various factors, from the way we are raised to who we associate ourselves with. When I think of what I have become, I think of all my external influences- what I have read, whom I have met during my lifetime, the experiences I have had; what I rarely ever think of is my genetic makeup and how it has influenced me as a person. Out of the functions that genes oversee in the human body, the most intriguing is sexual differentiation, which is the development of a person from an “undifferentiated zygote” to a fetus, which will then evolve into a walking, talking, conscious male or female (2).

Sexual differentiation occurs at conception; out of the 23 pairs of chromosomes from the parents, the 23rd pair, the sex chromosomes, ‘decides’ the sex of the unborn child. If the child receives an “XX” chromosome pair, it will be female; if the child receives an “XY” chromosome pair, it will be male. The “Y” chromosome is what makes a child male. If it is present, the sex glands, or gonads, become a penis; if it is absent, the zygote is by default female (1). This is because a gene in the short arm of the “Y” chromosome, referred to SRY, can be linked to a DNA-binding protein that causes “differentiation of cells derived from the genital ridges into testes.” In males, SRY, as well as other genes, start the process of differentiation by releasing Sertoli cells and, to a lesser degree, steroidogenic cells into Leydig cells; this results in the forming of the testes, which can begin to function 8 weeks into the gestation period. If SRY is not present, ovaries form in 2 to 6 months; in certain situations, ovaries do not form at all, which is due to the absence of the Xp and Xq genes (2).

Internally, sex differentiation is distinguishable within six to eight weeks of the gestation period. The wolffian and müllerian ducts are both formed by this period; one gland will flourish and one will shrivel up, which determines whether or not testosterone and AMH are produced. Testosterone will cause the wolffian glands to become the seminal vesicles, vas deferens, and epididymis; without the testosterone, the glands will disappear. The müllerian glands will develop into fallopian tubes, a uterus, and the upper region of the vagina, unless testosterone is present.

Externally, signs of genital differentiation are visible by as early as seven weeks, when the fetus develops a genital tubercle, urogenital groove and sinus, and labioscrotal folds. In most females, these parts become the clitoris, urethra, labia, and vagina. Males tend to develop slightly later than females; eight to ten weeks into the gestation period, their fetal genitalia turn into a penis, urethra, and scrotum. The external genitals do not fully develop in either sex until puberty, which occurs in the pre-teen and teenaged years (2).

In some instances, something goes astray in the forming of the sex organs and the balance of hormones. In the development of the internal sex organs, imbalances of the testosterone and AMH, as well as other chemicals called androgens, can lead to mistakes in the formation of the organs; in some cases of glandular development, the result can be the full development of both the wolffian and müllerian glands, or the development of neither. Either way, the lack of regulation in the glands creates a hormonal imbalance, which can result in an intersexual person. The general medical motto for sexual identity, anatomy-wise, is “The presence of a uterus is stronger evidence of absence of testes than the state of the external genitalia” (2); this means that the uterus, in essence, trumps the presence of male sex organs. A person with a uterus and testes is technically a woman, regardless of what sex the person looks like or identifies with. This is what can occur with gender identity disorder, which is more commonly known as transsexualism or transgender (2). Decades ago, it was thought that people learned the behavior of their sex from their surroundings; this would be the nurture part of the nature-nurture controversy. As time has passed, and more studies have been done, it has been shown that biology has a lot to do with how a person’s body develops, as well as how that same person feels about their body. Gender identity is not something that can be measured by scientists, but instead varies from person to person. There are as many different degrees of gender identity as there are people dealing with identity conflicts. These people are trapped in the wrong body; though they may appear to be of one gender, they completely associate themselves with the other. The nature part of the nature-nurture argument says that the “prenatal exposure to androgen influences the development of gender identity,” meaning that how a person is raised does not affect which gender they identify more with (3). An example of this would be a person who, due to some reason from fetal development, is born with both a uterus and internal testes. This person may identify more with the female that it appears to be, or with the other sex that it carries internally. This same example could happen with no conflicting sexual organs, and could simply just have a person that feels as if he or she was born in the wrong body.

While sexual differentiation is easy to explain scientifically, its effects are slightly more difficult to convey. The nature-nurture argument is almost impossible to prove or disprove one way or the other, because cases of conflicts in gender identity vary so much from person to person, while some cases are never even known. Gender identity remains an enigma, and it is doubtful whether its causes or effects will ever be fully known or understood.

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Luis Gutierrez's picture

zygote differentiation

How much time (however small) elapses between sexually undifferentiated and sexually differentiated zygote? Is this time measurable?