The Science of Gender and Sex

I saw an article this morning on CNN about Caster Semenya from South Africa and how she will be competing in the Olympics. Caster is the female runner who received a lot of media attention when here gender was called into question. She has been cleared for competition after some controversial gender testing. Today, I thought it would be good to write about the science of sex and gender. Who is a boy, who is a girl, and who unfortunately might be stuck somewhere in between.

For humans, sex is determined genetically. We have a set of sex chromosomes. If you are female, you have two X chromosomes, while if you are male you possess one X and one Y chromosome. Maleness is conferred by what is known as the SRY gene found on the Y chromosome. In fact, in mice, if you delete this region, the mice will be female. Ok, so that is what is considered “normal”. Now, let’s go through some changes that can result in some differences.

First, let’s go through sex chromosome abnormalities in humans. The abnormalities are caused by what is known as nondisjunction. When your sex cells divide, they are supposed to separate the sex chromosomes. So, for example, in females, each egg should only possess one X chromosome. In males, each sperm should only have one X or one Y chromosome. But sometimes, the sex chromosomes don’t separate, so in females you could get eggs that have two XX chromosomes, or even no X chromosomes. In males, you could get sperm that have an X and Y chromosome, or very rarely two Y chromosomes in the same cell.

Ok, so now that you know what nondisjunction is, let’s talk about the syndromes associated with this. The first on is called Klinefelter’s Syndrome. These individuals are XXY. These individuals are male because they have a Y chromosome. However, they usually don’t make as much testosterone as other males, so male teenagers with Klinefelter’s syndrome may have less facial and body hair and may be less muscular than other boys. These individuals can father children, but it may require hormone therapy at puberty.

Next is Turner Syndrome. This syndrome is the result of a female only having one X chromosome. Women are often shorter in stature and do not develop the secondary sex characteristics at puberty, such as breasts. Unfortunately, Turner Syndrome often causes  infertility. If a diagnosis is determined at the age of puberty, hormone therapy can help with development of the secondary sexual characteristics.

Triple X syndrome is when females have three X chromosomes. Usually, these females have no external features that indicates they have an extra X chromosome. They develop normally, though they may be taller, and the can conceive children. The same is true for men with XYY syndrome. The usually develop normally, but can be taller, and can father children.

Now I’ll discuss individuals who have normal sex chromosomes, but display sexual characteristics of the opposite sex. There can be XX intersex individuals. These individuals have the chromosomes and ovaries, uterus, and fallopian tubes of a woman, but have the external genitals of a male. This is thought to result from the fetus being exposed to excess male hormones before birth. These male hormones could be from the mother being exposed to testosterone, or ovarian tumors can produce male hormones.

There are also XY intersex individuals. These individuals are genetically male, but have external genitals that are incompletely formed, ambiguous, or are female. Testes may be present, malformed, or completely absent. This type of intersex can be caused issues with testes and proper testosterone levels.

Then there are individuals that have what is known at true gonadal intersex. These individuals have both male and female tissues. The causes for this are still not fully understood.

So, the point of this is not everything is black and white when it comes to being a certain sex. It’s complicated. I’ve heard of some unfortunate situations where children who were born intersex were raised as one gender and then really struggled because they thought that should be the other gender. Society likes to categorize people as one or the other, when the reality is there are also in betweens.


3 thoughts on “The Science of Gender and Sex

  1. Just an added comment: I’ve heard many people separate sex from gender, and I think it is a valid separation. Your sex is determined by your chromosomes and hormones. We do not yet know what all defines gender, and there are people who are XX who think, feel, and act like a male, and people who are XY who think, feel, and act like females. I think there are some cultural, sociological, and psychological aspects for gender that we have not (yet?) been able to give a genetic basis.

    • Good comment. My understanding of gender is basically what Julie said: it is how someone identifies rather than what their sex chromosomes determine. So someone with XY chromosomes is a male by sex but can identify as female or transgender instead. Excellent post Claire, concise and well explained.

  2. Excellent point Julie. There is a lot of mixing of the terms. All of the news articles about Caster point to her gender being called into question, which is why I labeled this post as about gender and sex. But, I think it was actually her sex being called into question, with people thinking she was male and therefore had a competative advantage. I totally agree that gender has more of a psychological basis rather than a genetic basis. I might have to do some more research into this for a future blog post!!

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