Klinefelter's syndrome, XXY males, can occur due to nondisjunction of X chromosomes during prophase of meiosis I in females.
One of the eggs from such a meiosis could receive both X chromosomes, and the other would receive no X chromosomes.
If these eggs are subsequently fertilized with normal sperm, various sex chromosome aneuploidies could occur:
XXY - sterile male who may have some female body characteristics, and in some cases increased learning hurdles.
(For more information visit the National Institute of Health's site on Understanding Klinefelter Syndrome, A Guide for XXY Males and their Families at http://www.nih.gov/health/chip/nichd/klinefelter/)
XXX - There are no real stigmata connected with this
syndrome. Women with triple X usually are fertile.
XO - sterile female with short stature, with low mental ability, failure to undergo puberty (Turner's syndrome).
YO - Non-viable; genes on X chromosome required for development and survival.
Nondisjunction during sperm production can also result in aneuploidy of sex chromosomes.