posted by Sarah .
how a person's cells could contain three sex chromosmes? XXY.
You might begin here:
What you are talking about is Klinefelter's syndrome, which occurs about once every 500-1000 births. He (a male because he has a Y chromosome) will typically have small external genitals, female secondary sex characteristics, reduced sex drive, erection problems, gender identity problems, be sterile, and have a greater likelihood of mental retardation.
Use the term for your search. You can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.
I hope this helps a little more. Thanks for asking.