This might sound like a silly question, but what chromosomes determine whether a human is born a boy or a girl? I ask this because I got https://embryo.asu.edu/pages/sex-determination-humans when googling the subject and it contains the word "typically" in places I did not expect. For example:

  1. Humans typically develop as either male or female

  2. Humans who inherit two X chromosomes typically develop as females, while humans with one X and one Y chromosome typically develop as males.

  3. Because females tend to only have X chromosomes, the egg cells that they produce typically carry an X chromosome

Is this a form of political correctness, or are there legitimate cases that justify the insertion of the word typically? I'm genuinely curious what these exceptional cases might be.


2 Answers 2


To answer points 1 and 2:

People born karyotypically male (XY) with complete androgen insensitivity syndrome (AIS), can be born without developed male genitals. People with a certain set of sex chromosomes develop in varying different ways.

To answer point three: An egg cell will indeed carry a single X chromosome in most cases, however, there are situations where an egg cell can carry two X chromosomes. If an egg cell containing two X chromosomes is fertilized with a sperm containing a Y chromosome then this could lead to XXY chromosomes — Klinefelter syndrome.

Side notes:

For those interested in the impact of social standards regarding this syndrome, there is a good interview with a Dutch person with AIS. (One can set the translated captions to most languages.)

You can find information on the occurrence rate of intersex here: Intersex Society of North America

  • 1
    $\begingroup$ Very interesting! Thank you very much. $\endgroup$
    – Gili
    Commented Feb 10, 2023 at 19:15

There is a a whole mess of inhibitors and activators proteins that need to be triggered for typical sexual development, a mutation or error in any one can lead to atypical results even if they have typical chromosomes. A graphical overview is below. You may note than several factors are common to both sexes. Atypical can range from just malformation of genitals to full on sex reversal. XY female and XX male exist. Then you have all the atypical chromosome combinations, Xo, XXX, XYY, XXYY, ect. A full breakdown of most the known options (as of 2016) can be found here. And that's just the gross physical manifestations, the brain has largely separate pathways for sex determined development, meaning an alteration of those pathways can lead to what is essentially a male brain in a female body and vice versa.

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