Sex vs. gender: the basics

Sex determination, assigned sex

First of all, let’s get acquainted with the terms. “Sex” and “gender” are not the same, although people often use the words as though they mean one thing. 

“Sex” is about biological characteristics. “Gender” is about expectations we place on different people in society. Soon we’ll dive deeper into the intricacies of the latter; in the first part of this topic, however, we’ll focus on the former—namely, on how people “get” their sex.

For many people, sexes seem pretty straightforward: there are males, there are females, and that’s it. Science, however, begs to disagree. 

The biological characteristics of sex come down to a lot of physical and physiological factors: chromosomes, gene expression, hormone levels and functionality, and reproductive anatomy (which includes sex glands—also called gonads—and external sex organs).

People are commonly determined to be of one or the other sex at birth: a doctor assigns the sex, looking at the baby’s external sex organs (also called genitalia or genitals). While most of the time the process is accurate, sometimes the genitals mismatch other sex and gender characteristics.  That’s why some people prefer to say “assigned sex” instead of “biological sex”: there’s much more to biology than just the genitals.

The sex the person is assigned to is written down in their birth certificate. Most likely, their assigned sex has been determined nine months before the birth: at fertilization, when the sperm was united with the egg.

Most of our genetic information is kept in long DNA molecules called chromosomes. Allosomes—sex chromosomes—are special ones that have all the information related to our sex. A human usually has a pair of sex chromosomes, receiving one from the mother and one from the father.

There are two kinds of sex chromosomes: X and Y. Female sex is normally assigned to a person with a pair of X’s (XX), male sex—to a person with XY.

So, back to the fertilization: each sperm cell can carry within either an X or a Y chromosome—the sperm comes from a male, who has both. An egg cell can have only an X chromosome. When the sperm and the egg unite, these chromosomes combine to create either an XX or an XY pair.

This composition of chromosomes is called the chromosomal sex—and most often it corresponds to the assigned sex. Doctors can’t look at the chromosomes directly, but the sex chromosomes determine which reproductive parts the baby gets.

It happens at about five weeks into the embryo’s development. At first all embryos, regardless of the sex chromosomes, have the same tissues—as well as the potential to develop either male or female anatomy. Around the five-week mark, the changes begin.

Sex development and differentiation

Let’s look at how genes and hormones promote sex differentiation. 

We already know that five weeks is an essential moment in sex development. At this time, the embryo develops gonadal ridges—two tiny bulges alongside the kidneys. These bulges will become testicles or ovaries in the future.

Next to them, there are two sets of ducts. Müllerian ducts may develop into the uterus and fallopian tubes, which are the female reproductive organs. Wolffian ducts have the potential to become the internal male reproductive structures: the epididymis, vas deferens, and seminal vesicles.

The outcome is influenced by a multitude of genes, each “pushing” in different directions, towards the male or the female route of development. It usually ends with one of the gene sets “winning” and blocking the other route.

For example, the SRY gene located on the Y chromosome starts the development of the testicles. Later, the testicles will start producing androgens (male sex hormones), like testosterone, AMH, and DHT. The hormones in turn promote the formation of the male ducts. At approximately nine weeks, these hormones also support the development of the penis.

On the other hand, WNT4 is a gene that actively promotes female sex development, suppressing the male route. If the ovaries are formed, they produce estrogens and progestogens — the female sex hormones, which in the absence of testosterone promote the development of the female internal and external sex organs.

Years after birth—at puberty—the sex hormones come into play again, inducing the development of secondary sex characteristics such as breasts or facial hair.  A recent discovery has revealed that the mother and the child may swap a tiny portion of their cells during pregnancy! If the baby is a boy with XY chromosomes, the mother (whose own cells are XX) can end up with up to 1 in 1000 cells being XY. These “other” cells can build themselves into any of her organs and live up to several decades afterward. This phenomenon is called “microchimerism,” and its implications are still unclear. Some suggest that the baby’s cells can speed up healing processes in the mother’s body; others point out the increased risk for autoimmune diseases and cancer. The reverse process—when the baby receives the mother’s cells—happens far less often.

Let’s recap the process of sex differentiation as it usually happens:

• During fertilization, the X/Y chromosome from the sperm cell is combined with the X chromosome in the egg.
• The XX or XY pair determines the chromosomal sex.
• At five weeks, the genes encoded in the chromosomes promote the development of the gonadal ridge into gonads: testicles or ovaries.
• These gonads produce sex hormones.
• At nine weeks, the sex hormones promote further changes to external and internal sex organs.
• At puberty, the hormones influence the development of secondary sexual characteristics.

Intersex

Just by looking at a simplified recap of sex determination and differentiation, you probably agree that it’s quite an intricate process. What’s more: it doesn’t stop with birth (when the sex is assigned).

There are many possibilities for things to deviate from the predetermined route. When someone’s sex development doesn’t fit the typical definitions of “male” or “female,”—this person may be described as intersex.

A more medical-sounding term for intersex variations is “Differences in Sex Development,” or DSDs. However, not every intersex person may be comfortable with this term.

The estimation of how many intersex people are there largely depends on what you choose to define as intersex. In the most strict definition, around 0.05% of all people (about one in two thousand babies) are born with noticeable differences in external sex organs.