Klinefelter's syndrome is a genetic condition that only affects males. The condition is present from birth and is due to an extra $X$ chromosome. Normal males have $45+X Y$ chromosomes but males with Klinefelter's syndrome have $45+$ XXY chromosome. This condition arises due to non-disjunction at the time of gamete formation leading to formation of abnormal gamete. When either a normal egg fuses with abnormal sperms (XY) or an abnormal egg (XX) fuses with a normal sperm it results in an abnormal zygote, which develops into a boy with the syndrome.