The science says humans are generally born with 23 sets of chromomes for a total of 46 and that 'atypical' females possess XX sex chromosones while 'atypical' males possess XY. It also says that alterations can and do occur to such due to mutations, much in the same way that chromosomal defeats and/or translocations exist within individuals born with particular physical and/or mental handicaps such as Down's Syndrome, etc, which is caused by possessing an extra chromosome.
As such, people can be born born X (Turner Syndrome), XX (atypical female), XXX (Trisomy), XY (atypical male), XXY (Klinefelter Syndrome), XYY, etc. and sometimes even a combination of XX and XY (Mosaics) most commonly known as 'intersex'.
It doesn't take a rocket scientist to recognize that one's genetic makeup can and does affect one's physical, mental, psychological and/or emotional health and that sex/gender are not as simplistic contructs as some would have you believe.