A chromosome is a DNA molecule that contains genetic material. Sex chromosomes are chromosomes that affect your sexual anatomy and reproductive development.

Most of us were raised with the idea that there are two sexes: male and female. We’re usually told that people with XY chromosomes are male and people with XX chromosomes are female.

It’s a lot more complicated than that.

Typically, there are 46 chromosomes in each human cell. This usually includes a pair of sex chromosomes and 22 pairs of autosomes. Autosomes are all chromosomes that are not sex chromosomes.

Many people are born with variations in sex chromosomes, also called X and Y variations or sex chromosome aneuploidy (SCA).

People with an SCA may have one X chromosome, XXY chromosomes, XXX chromosomes, and so on.

Sex chromosomes typically determine the development of your reproductive organs and your secondary sex characteristics.

For example, your sex chromosomes may determine:

  • the hormones your body produces
  • whether you have ovaries or testes
  • the development of breast tissue

Having certain sex chromosomes may also make you more susceptible to specific health conditions. For example, hemophilia A and B are sex-linked conditions more likely to appear in people with XY chromosomes.

Primary sex determination depends on chromosomes. Secondary sex determination depends mostly on hormones.

Primary sex characteristics include the gonads, which are the glands that produce reproductive hormones in the body.

Secondary sex characteristics are determined by the hormones the gonads secrete. These characteristics include your body’s phenotype (how it looks) outside of the reproductive system.

Examples of secondary sex characteristics include:

  • enlarged breasts
  • wider hips
  • facial and bodily hair
  • Adam’s apples

Many people have a difference in sexual development (DSD), which is also called intersex. “DSD” is a term that describes chromosomes, sex characteristics, and anatomy that can’t be defined as exclusively male or female.

As many as 1 in 100 people are born with a DSD.

Not everyone with a DSD has variations in sex chromosomes. Some people with DSDs may have XX or XY chromosomes. But they might also have X and Y variations or SCA.

Variations in sex chromosomes can include:

Different SCAs produce different characteristics.

How many sexes are there?

If you look this up, you’ll undoubtedly find a variety of opinions on the matter. Some say there are only two sexes, while others say there are an infinite number of sexes.

It depends on your definition of sex: What makes a specific phenotype a “sex”? How do you determine the difference between a sex category and a sex variation? It’s hard to say.

Many biologists assert that sex is a spectrum and not a binary, meaning there are many variations in sex and not simply two exclusive categories.

Gender and sex are different things — but gender is also not a binary.

How many people are born intersex?

As many as 1 in 100 people are born with a difference in sex development (DSD), which is another term for intersex.

According to the Intersex Society of North America, about 1 in 1,500 to 1 in 2,000 people are determined intersex at birth.

However, some intersex people only find out about their DSD later on in life, like at puberty or as an adult. It’s even possible to have intersex traits and never know.

This number depends on how you define “intersex.” Many people have variations in sex differences that are not apparent to others. As such, some clinicians might not consider them intersex.

What are karyotypes?

Your karyotype is your complete set of chromosomes. A karyotype is used to visualize the appearance of your chromosomes, including the size, number, and shape of the chromosomes.

Sex chromosomes determine the development of your gonads. But sex chromosomes are a lot more complex than most people think.

While we’re typically taught that there are only two sex chromosome types — XX and XY — the truth is there are more variations than that.

Sian Ferguson is a freelance health and cannabis writer based in Cape Town, South Africa. She’s passionate about empowering readers to take care of their mental and physical health through science-based, empathetically delivered information.