In a developing fetus, what influences whether it is going to become male or female? What influences the cells to start building male or female organs?
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In mammals, egg cells from females contain an X chromosome, and sperm cell from males have either an X or Y chromosome. When they come together, the result is a cell that has either XX or XY (usually). That cell keeps dividing and develops into the baby.
There’s a gene called SRY that’s usually on the Y chromosome. It causes cells to make a protein that grabs onto DNA in a way that turns on a series of other genes that trigger modifications to anatomy that turn the animal male. The most important gene it turns on in the process is called “testis determining factor”, and that signals cells making sex organs to switch to the male pattern. It’s quite complicated, but flipping that one switch changes hormones and genetic activity that alter the development of the embryo.
Because SRY is on the Y chromosome, animals that have two X chromosomes are usually female, and those with an X and Y are usually
male. There are instances where the SRY gene breaks off and becomes part of an X chromosome, which gives us XX males. Also sometimes there are mutations in the SRY gene or other genes it interacts with so it doesn’t work and you get XY females (or intersex individuals).
Interestingly, precisely what genes SRY turns on differs among mammals.
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