depends what sperm gets to the egg. its random wether each sperm has the X or Y chromosome.
the woman’s egg always has an X chromosome.
when they combine together, you have only 2 choices – XX or XY
(besides some uncommon anomalies where there is a different combination, this for example is how down syndrome happens)
if its XX, its a girl, if its XY its a boy
What others have said regarding chromosomes is not technically correct.
Male/female expression is controlled via the existance of testosterone, as detected by Androgen Receptors.
The vagina and the penis are built from the same part of the body. Ovaries and testicles are also the same proto-organ. Androgen receptors detect testosterone and trigger the gonads to either turn into ovaries, or descend and become testicles. Similarly, the proto-penis will either grow and the womb gets reabsorbed, with parts of it being used in fabrication of certain connections in the penis, or the proto penis will invert, becoming the vagina, clitoris, etc and connecting to the womb, while the tubes connect the womb to the overies.
It is the Y chromosome that is generally considered to code for the production of testosterone.
Source: I was born with persistent mullarian duct syndrome as a result of androgen receptor deficiency.
Results: womb wasn’t entirely reabsorbed. testicles didn’t descend but did turn into testicles, though being internal caused them damage and I am sterile as a result, and on the smaller side of average.
Oi. The XY chromosome thing is a simplified explanation from 6th grade science. The reality is its MUCH more complex. Scientific American published an article just touching on the complexities of [sex and gender development]([https://blogs.scientificamerican.com/sa-visual/visualizing-sex-as-a-spectrum/](https://blogs.scientificamerican.com/sa-visual/visualizing-sex-as-a-spectrum/)) in 2017 that touches on it and also offers a visual of just how many moving parts go into it and where they can diverge.
The DNA determines sex. The father either provides an X or a Y chromosome which pairs with an X provided by the mother. XY makes a male, XX makes a female. In the case of a male, the Y chromosome is present which carries a gene on it called SRY, which around 6-8 weeks causes a protein called TDA (testicle determining factor) to be produced. This protein signals the gonads (which at this point could become ovaries or testicles) to become testicles. Then, the testicles will begin to make testosterone which will trigger a bunch of other things to happen that result in a male child. They also make another hormone that blocks female traits from developing. Lacking a Y chromosome, none of these things happen and the child continues to develop as female.
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