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Initial sex in placental mammals is determined to be male if the individual is 46XY (the karyotype is normal with an X and a Y chromosome) or female if the individual is 46XX (the karyotype is normal with two X chromosomes).

Every cell of the individual will contain the full set of chromosomes. The sex-determining region of the Y chromosome appears to contain the trigger that sets in motion the events that lead to a male organism. Otherwise, development leads to a female.

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Excerpted from: Genetics Lecture 4