X and Y chromosomes
Why do X and Y chromosomes determine a persons sex?
Human females contain two X chromosomes, and males contain one X and one Y
chromosome. In the absence of a Y chromosome, a human fetus will develop as a female.
This indicates that a gene or genes on the Y chromosome determines "maleness". Several
genes that are required for male development have been located on the Y chromosome, but
as far as I know, the "master" gene has not yet been identified.
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Update: June 2012