You may have learned in school that people are either XX (female) or XY (male).
But the biology of sex — and how it’s determined — is more layered than that.
Let’s walk through what actually happens at the genetic level, and how we talk about it in care.
🧬 Chromosomes and sex: the basics
Humans typically have 23 pairs of chromosomes. The 23rd pair — the “sex chromosomes” — usually determines biological sex.
XX → typically leads to female sex traits
XY → typically leads to male sex traits
But this isn’t the full story.
🔄 Variations that occur naturally:
XXY (Klinefelter Syndrome):
Individuals have an extra X chromosome. May appear male at birth but have lower testosterone and may experience breast development or infertility.XO (Turner Syndrome):
Only one X chromosome. Individuals typically present as female but may have delayed puberty, short stature, or infertility.XYY Syndrome:
Extra Y chromosome. Often taller than average, may or may not notice any differences.XX Male / XY Female (Swyer Syndrome, Androgen Insensitivity Syndrome):
Due to gene-level variations or hormone receptor issues, someone may have the chromosomes of one sex but develop physical traits of another.
These are all part of normal human diversity — and don’t make a person “less” anything.
💡 So what do AMAB, AFAB, and intersex mean?
AMAB = Assigned Male at Birth
Someone born with physical traits typically associated with male anatomy.AFAB = Assigned Female at Birth
Someone born with physical traits typically associated with female anatomy.
These terms help us talk about medical care without assuming gender identity.
🧬 Intersex
Intersex is an umbrella term for people born with:
Chromosomes
Gonads
Hormones
Genitalia
…that don’t fit typical definitions of “male” or “female.”
This can be apparent at birth, during puberty, or discovered later through labs or genetic testing.
Intersex traits are more common than many realize — affecting up to 1.7% of the population, which is as common as having red hair.
🧠 Why this matters in healthcare:
Understanding your genetic sex helps us:
Interpret hormone labs accurately
Assess cardiovascular and bone health risk
Screen appropriately for reproductive cancers
Prescribe medications with proper dosing and metabolism in mind
It also helps us hold space for gender diversity, and avoid assumptions based on anatomy alone.
📌 The takeaway:
Genetic sex is shaped by chromosomes — but it’s not always binary.
Biology and identity are both real, complex, and valid.
And at Fishtown Medicine, we hold that complexity with respect and curiosity — not judgment.
Want to learn more about your own genetics or hormone pathways? We offer precision testing with counseling and context to help you make sense of it all.