By Jean-Michel Fustin,, The Conversation

Credit: Katie Rainbow 🏳️‍🌈 from Pexels

I teach a university course called Hormones and Behaviour. What surprises students is that biological sex is not produced by a single switch. What surprises me is that many students have never been taught this before. It is basic biology, but it helps explain life.

Sex emerges through a sequence of events before birth and continues at puberty. Chromosomes help direct the formation of gonads, the body parts that usually become ovaries or testes. Gonads make hormones. The body then has to respond to those hormones. In most people, these steps flow in the same direction. But in rare cases, they do not.

These cases are known as disorders of sex development, more recently called differences of sex development (DSDs). They are not new, but they provide a useful way to understand how bodies develop. DSDs can affect hormone production. Some affect the body's response to hormones. Some affect chromosomes or gonad development. Two people can have a DSD yet have very different bodies, medical needs and life experiences.

Each condition reveals what can happen when one step in the normal sequence is altered. Here are five ways sex development can take a different path.

This DSD affects about1 in 16,000 births.

The adrenal glands, which normally produce small amounts of male hormones, even in females, are hyperactive in this condition.

A baby with two X chromosomes and ovaries can therefore be born with external genitalia that look male.

In contrast, a child with complete androgen insensitivity syndrome (CAIS) can have XY chromosomes and internal testes that make testosterone, but their body cannot respond to that testosterone.

This lack of a response to testosterone keeps the developing fetus in its "default" female state, leading to female external genitalia at birth.

Someone with CAIS will develop breasts at puberty. Since their external anatomy is typically female, people with CAIS areusually raised as girlsand may only be diagnosed later, often when their periods do not start.

CAIS is estimated to affect1 to 5 in 100,000 live-born females.

Another condition, called5-alpha-reductase deficiency, shows how sex development does not finish at birth.

The body needs an enzyme called 5-alpha-reductase to turn testosterone into a stronger version of the hormone before birth. When a genetic mutation inactivates this enzyme, an XY baby may be born with genitalia that are read as female and may be raised as a girl. At puberty, however, the normal surge of conventional testosterone masculinizes the body.

The condition is rare worldwide, and its exact incidence is unknown, partly because conditions affecting sex development have often been surrounded by silence.

In parts of the Dominican Republic, where this condition is known asguevedoces, studies have reported surprisingly high local rates. A rare genetic mutation is more common in these isolated populations, affecting about1 in 90 males.

More recently, some of these communities have begun to recognizeguevedocesat birth and raise them as boys.

Usually, females receive one X chromosome from their father and one X from their mother and are therefore XX, while males receive one X chromosome from their mother and the Y chromosome from their father. However, a person withKlinefelter syndrome, affecting1 in 500–1,000 male births, hasXXY chromosomes, causing smaller testes, scant body hair, lower testosterone and reduced fertility. It is often never diagnosed.

On the other hand, Turner syndrome, or the loss in females of one of the two X chromosomes, affects1 in 2,500 female birthsand impairs growth, puberty and fertility. It canaffect physical appearance, for instance causing a wide neck and broad chest, and often comes with health conditions such as hearing loss and kidney problems.

Swyer syndrome is rarer: A person has XY chromosomes but a mutation in the Y chromosome disrupts the signal that normally drives testis formation. The gonads do not develop into testes, testosterone remains low and the external and internalanatomy is typically female.

These are the clearer examples. Milder sex differences can result from mutations that reduce, rather than completely inactivate, an enzyme or hormone response. For example,non-classic congenital adrenal hyperplasia, where some enzyme activity remains. Their effects may be less obvious, and their incidence is often unknown because many cause subtler changes or are never diagnosed.

None of this is new. These conditions have been known to scientists for decades. But nowgenetic testingcan identify the mutation behind some cases.

Specialist clinics and patient groupshave pushed for clearer information for parents, better psychological support for patients and greater caution around irreversible childhood surgery when it is not medically urgent. Recentpatient-centered researchshows that clear information, good communication and psychological support are central to good care for people with DSDs. It's unclear whether increased awareness has improved people's lives, but the direction is clear: better information, specialist care and long-term support matter.

The lesson is not that sex is meaningless or endlessly fluid. These rare conditions do not provide a simple answer to every argument about sex and gender.

However, DSDs do show sex is not a single event. Just like the growth of our bodies, it is a process. When chromosomes, gonads, hormones and the body's response to them all follow the usual sequence, the result often looks simple. When one link changes, the body may no longer fit the binary pattern many of us were taught at school.

For the people affected, this is not an abstract puzzle. Some need lifelong hormone treatment. Some face infertility. Some discover their condition only as teenagers or adults. Chromosomes or sex designation at birth may explain part of the biology behind DSDs. They do not explain what it feels like to grow up, go through puberty, seek fertility treatment or decide who needs to know.