Beyond XX and XY – The Spectrum of Sex

Beyond XX and XY – The Spectrum of Sex

The Impossibility of Defining Sex: A Complex Interplay of Chromosomes, Hormones and Anatomy

Defining sex is not straightforward due to the complex interplay of chromosomes, hormones, and anatomy. This complexity is further complicated by conditions like androgen insensitivity syndrome (AIS), which complicates the lines of traditional sex categorisation, challenging our understanding of sex and making the topic all the more intriguing and engaging.

Sex is typically determined by sex chromosomes, where females usually have two X chromosomes (XX), and males have one X and one Y chromosome (XY). The Y chromosome is crucial for developing testes, which produce androgens, hormones that facilitate the development of male sex organs.

However, Androgen Insensitivity Syndrome (AIS) is a known medical condition that blurs the definition of sex. In AIS, a person who is genetically male (XY) is insensitive to androgen. Their body either ignores androgen or is insensitive to it, resulting in the development of genitals along female lines, with the testes typically remaining inside the body.

AIS is categorised into two types based on the level of androgen insensitivity:

• In people with Complete Androgen Insensitivity Syndrome (CAIS), there is total insensitivity to androgen, and the individual develops external genitals that are entirely female.
• In people with Partial Androgen Insensitivity Syndrome (PAIS), there is some androgen sensitivity, and the level of sensitivity determines how the genitals develop. Depending on the level of insensitivity to androgen, individuals may appear almost entirely male, entirely female, or have both male and female characteristics.

In 2015, the Daily Mail reported the story of Hayley Haynes, who was born with AIS and was told by doctors at 19 that she had XY chromosomes, which genetically made her male. She was born without a womb, ovaries, or fallopian tubes. Hayley’s case demonstrates how someone with XY chromosomes can have female characteristics due to androgen insensitivity.

However, Hayley was able to give birth to twin girls through IVF treatment using an egg donor after doctors discovered she had a tiny womb that had been missed on previous scans. She underwent hormone therapy to create an environment in which her womb could grow. This case demonstrates that medical advancements can enable individuals to overcome biological limitations traditionally associated with their sex, further complicating the definition of sex.

There are other medical conditions that make defining sex incredibly difficult.

• Congenital Adrenal Hyperplasia (CAH): This genetic condition can cause individuals with XX chromosomes (typically female) to have higher levels of male hormones. This may result in ambiguous genitalia or characteristics that are more typically male.
• Klinefelter Syndrome: Individuals with this condition have an extra X chromosome (XXY). They are typically assigned male at birth but may develop some female physical traits during puberty.
• Turner Syndrome: Individuals with Turner Syndrome have only one X chromosome (X0). They are typically assigned female at birth, but may have underdeveloped female sex characteristics.
• 5-Alpha-Reductase Deficiency: This condition affects the conversion of testosterone into dihydrotestosterone (DHT), a hormone critical for male development. Individuals with this condition may be born with ambiguous genitalia and may experience masculinization during puberty.

In 2019, an article in Science America concluded:

It is time that we acknowledge this. Defining a person’s sex identity using decontextualized “facts” is unscientific and dehumanizing. The trans experience provides essential insights into the science of sex and scientifically demonstrates that uncommon and atypical phenomena are vital for a successful living system. Even the scientific endeavour itself is quantifiably better when it is more inclusive and diverse. So, no matter what a pundit, politician or internet troll may say, trans people are an indispensable part of our living reality. Transgender humans represent the complexity and diversity that are fundamental features of life, evolution and nature itself. That is a fact.

In 2018, the same source concluded that the idea of only two sexes was “overly simplistic.”

In January 2025, President Trump made an executive order saying there were only two sexes. Scientists quickly responded, as reported by the Guardian saying:

While sex has traditionally been viewed as strictly binary, many experts now believe it is more complex and goes beyond a strict two-category system. The likes of Nature, possibly one of the most prestigious scientific journals in the world, has noted that “the research and medical community now sees sex as more complex than male and female”. And there’s a huge amount of disagreement as to how these categories should be described. “Scientists ourselves cannot agree on how to define the two sexes,” Rachel Levin, a professor of biology and neuroscience at Pomona College who studies the development of sex, told me over the phone. “To say that sex is simple and easily defined – and defined at conception – is factually incorrect.”

There are many variations to prove the biological diversity of sex.

It’s important to note that sex is not solely defined by external genitalia; it includes a combination of chromosomes, hormones, internal reproductive organs, and secondary sexual characteristics. This complexity is why defining sex can sometimes be challenging, if not impossible.