The intersex symbol nito/ShutterstockI 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, more recently 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. Congenital adrenal hyperplasiaThis DSD affects around one 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 genitals that look male. Complete androgen insensitivity syndromeIn 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 genitals at birth.Someone with CAIS will develop breasts at puberty. Since their external anatomy is typically female, people with CAIS are usually raised as girls and may only be diagnosed later, often when their periods do not start. CAIS is estimated to affect one to five in 100,000 live born females.5-alpha-reductase deficiencyAnother condition, called 5-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 genitals that are read as female and may be raised as a girl. At puberty, however, the normal surge of conventional testosterone masculinises 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 as guevedoces, studies have reported surprisingly high local rates. A rare genetic mutation is more common in these isolated populations, about one in 90 males. More recently, some of these communities have begun to recognise guevedoces at birth and raise them as boys. A person’s sex is more complicated than you might think. s880/Shutterstock Sex chromosomes disordersUsually, 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 with Klinefelter syndrome, affecting one in 500-1,000 male births, has XXY 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, affects one in 2,500 female births and impairs growth, puberty and fertility. It can affect physical appearance, for instance 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 with a mutation in the Y chromosome that disrupts the signal that normally drives testes formation. The gonads do not develop into testes, testosterone remains low and the external and internal anatomy 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.What this does and does not tell usNone of this is new. These conditions have been known to scientists for decades. But now genetic testing can identify the mutation behind some cases.Specialist clinics and patient groups have pushed for clearer information for parents, better psychological support for patients and greater caution around irreversible childhood surgery when it is not medically urgent. Recent patient-centred research shows 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 body, 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 behinds DSDs. It does not explain what it feels like to grow up, go through puberty, seek fertility treatment or decide who needs to know.Jean-Michel Fustin is a Medical Research Council-funded Senior Research Fellow in the Division of Integrative Physiology at The University of Manchester. He contributes the level-3 course Hormones & Behaviour.