Let’s talk about sex

Allie White
Oct 1
8 min read

Writer: Allie White

Editors: Sam Alper, Sarah Brockway, Sophia Andreadis

Illustrator: Maddie Swall

Illustration by Maddie Swall

The Beginning

It’s a tale as old as time. When a sperm loves an egg very much, the two join together to create a beautiful baby cell. This cell grows into an embryo, and, eventually, a fetus. When this fetus enters the world, we assign it a sex – either ‘male’ or ‘female’ – based exclusively on its external features. Historically, it was believed that sex is biologically binary: that there are two boxes and each human must fit into either one or the other. But when we dive deeper into the science behind sex determination, differentiation, and development, we find that the lines between ‘male’ and ‘female’ start to blur.

There are three main factors that doctors and scientists typically use to define an individual’s sex: genetics, hormone production, and hormone response. These three characteristics, however, don’t necessarily align with each other to point towards one sex or the other. During a person’s development, changes within and between each of these categories can occur that are outside of our “normal” definition of sex. It is estimated that 1 in 90 people are intersex, meaning they live in a body that does not match current definitions of either ‘male’ or ‘female’.

So, in what ways might someone exist outside the binary we’ve created?

Genetics

Let’s start at the beginning with genetics. When a sperm fuses with an egg, a new cell is formed. This cell inherits 23 chromosomes from each parent. These chromosomes carry the genetic instructions for the development and function of the individual. The 23^rd chromosomes contain information about sex development and each chromosome in this pair can either be an X chromosome or a Y chromosome. We typically define individuals who inherit two X chromosomes (XX) as ‘female’, and individuals who inherit one X chromosome and one Y chromosome (XY) as ‘male’.

This definition assumes that every individual will always inherit two sex chromosomes, either XX or XY. However, this isn’t the case. In reality, there are a number of combinations of sex chromosomes that one can inherit: XXX, XXY, XXYY, XYY, and XO (in which a person inherits only an X chromosome). This can happen due to a variation in the formation of an egg or a sperm. For example, sometimes when the sperm and egg fuse, an extra chromosome can sneak in or be left out, resulting in an individual with more or less than two sex chromosomes. This can often go completely unnoticed unless someone undergoes specific genetic testing because individuals with these variations can often look like others of their assumed sex. Individuals who do not inherit either XX or XY exist outside of the sex binary, suggesting that our definition of genetic sex as only ‘male’ (XY) or ‘female’ (XX) is flawed.

Even the cells in our body that are not involved in sex development and differentiation inherit X and Y chromosomes. It is commonly believed that each of the cells in our bodies has the same genetic composition, meaning each cell has the same 46 chromosomes that we originally inherited from our parents. However, it has recently come to light that this isn’t always the case. There are three phenomena that challenge this notion: mosaicism, chimerism, and microchimerism. Similar to a stained-glass window, in which glass pieces of all different shapes, sizes, and colors come together to create a beautiful, cohesive picture, cells with different genetic makeups can come together to form one individual.

Mosaicism and chimerism occur when a person has genetically different cell populations making up their body. Mosaicism can happen when a genetic mutation occurs during development that affects some cells but not others. When this mutation occurs in the sex chromosomes, it can result in different cells having different chromosomal makeups. Chimerism happens when two fertilized eggs merge in the womb. The result is an individual that has cells that were once part of another embryo. These cells can have completely different chromosomes than the cells from the other embryo.

These conditions can go completely undetected unless an individual undergoes genetic testing. This happened to a 46 year old pregnant woman when she visited Paul James’s clinic to screen the fetus for genetic abnormalities. The results showed that the baby was healthy, but the testing revealed that the woman’s body was made up of two chromosomally distinct populations of cells – some ‘female’ (XX) and some ‘male’ (XY). This woman developed and presented in all other ways as ‘female’, so for the first 46 years of her life she never knew that a large portion of her cells are what we would consider chromosomally ‘male’.

Finally, microchimerism, a common form of chimerism, occurs when a fetus swaps cells with its parent while in the womb. In this scenario, both the parent and child end up with each other’s cells. Scientists have learned that these cells can continue to live in both the parent and child for a long time after the initial swap. So, a person can have cells in their body with their parent’s or child’s chromosomes, even if those chromosomes are completely different from their own.

With our current definition of sex, some cells in individuals with these conditions would be considered ‘male’ and others ‘female’, leaving us with quite the paradox. These situations occur naturally, are incredibly common, and may go completely undetected since they don’t always affect external appearance. This again suggests that our binary idea of sex does not encompass all of the biological variations that exist.

So, if chromosomal and cellular makeup is not a sure way to infer one’s sex, what about hormones?

Hormone Production

The next step in development after chromosomal inheritance is sex differentiation, which typically begins five weeks after fertilization. Sex differentiation refers to the processes by which an organism develops testes or ovaries, the primary sex organs. Many different hormones play a role in this process.

Testes (typically thought of as ‘male’ sex organs) and ovaries (typically thought of as ‘female’ sex organs) both produce the same group of hormones: androgens. The key difference between these primary sex organs is the amount of androgens they convert into estrogen or testosterone. Typically, those assigned ‘male’ at birth are thought to convert most of these androgens into testosterone, and those assigned ‘female’ at birth are thought to convert most of these androgens into estrogen. But this isn’t always the case.

Each body is unique in the way it produces and processes hormones, resulting in a vast amount of variation in sex characteristics. In some cases, individuals don’t develop just one type of sex organ, but develop both testes and ovaries at the same time. Other times, individuals develop neither of these primary sex organs. This can cause wide variations in hormone production. Even individuals who develop just testes or just ovaries can have a mismatch between their chromosomal makeup and their internal sex organs due to differences in hormone conversion. For example, individuals with XX chromosomes can develop testes if they lack or have a mutation in a certain gene that causes them to convert more androgen into testosterone instead of estrogen. This leads to the production of internal and external sex organs typically thought of as ‘male’, even though the person has what is thought of as a ‘female’ (XX) chromosomal makeup.

These variations in sex differentiation caused by the fluidity in the types and levels of hormone production allow for various combinations of sex characteristics that do not follow a typical binary pathway. Take Caster Semenya, for example. Semenya is a world-record holding track star who was subjected to ‘sex testing’ when complaints started flooding in from her competitors saying she is too fast and, therefore, must be a man. Her test results showed that Semenya had high levels of testosterone and internal testes, despite her external sex characteristics appearing ‘female’. Semenya was then temporarily banned from her sport and told she could come back only if she took drugs to suppress her testosterone production and undergo surgery to remove her testes. However, there is no viable scientific justification for her exclusion from the sport or these demands. This blatant discrimination completely ignores the fact that plenty of individuals have some sort of blend of both ‘male’ and ‘female’ characteristics, just like Semenya. In order to compete in sports, people are forced to adhere completely to our current definitions of ‘male’ or ‘female’, which leads to discrimination against all those who exist outside this binary.

Clearly, no sex binary exists when it comes to hormone production, but what happens when we look at the way the body responds to these hormones?

Hormone Response

Even if the body produces all the “right” levels and kinds of hormones at all the “right” times, the body might not respond to these hormones in the way we expect it to. There are two issues that can occur that disrupt an individual’s response to the hormones that influence sex characteristics. First, hormones must bind to receptors to exert their effects. Mutations in these receptors may cause certain hormones, like testosterone or estrogen, to become inactive or non-functional. Second, the body might have trouble converting or breaking down certain hormones into their active components, like how testes and ovaries need to convert androgens into testosterone or estrogen for these hormones to have their intended effects.

People with these types of developmental variations are typically assigned one sex or the other at birth based and often undergo surgery before the age of two in order to align their anatomy with our conception of what ‘male’ and ‘female’ bodies should look like. These medical procedures usually have no health benefit other than making an individual better conform to a “normal” sex classification. Given the innate nature of hormonal variations during sex development, our concept of “normal” when it comes to sex should be expanded to include intersex individuals instead of forcing them, without consent, to comply with the preconceived notions of sex currently held.

The Sextrum

Given all the variations that occur in the conglomeration of factors that determine a person’s sex, it seems only reasonable to conclude that there is not a binary when it comes to sex. Instead, sex exists more on a spectrum. There is no clear and strict divide between ‘male’ and ‘female’. Based on an individual’s genetics, hormone production, and hormone response, a person might have some characteristics that we think of as more typically ‘male’ and others that we think of as more typically ‘female’.

Biology isn’t perfect. We are made up of billions of cells all acting on precise timescales, responding to specific signals, and producing unique responses to keep us alive. With all of these processes constantly working together in unison to create life, it’s not surprising that everything doesn’t always go exactly the same way every time.

We are constantly learning new information about how the body works and with this new information we need to update our definitions of our own biology. Knowing what we do now about the development of sex, it’s important that we stop putting people into boxes that they do not fit into. We must broaden our definitions and understand that, regardless of what chromosomes we have, what cells compose our bodies, or which hormones we produce or respond to, we are all humans. We all deserve to be seen and treated as equals no matter how our bodies have developed.