Underlying Causes of Klinefelter's Syndrome.
The usual pattern chromosomal pattern in a "normal" man is 44 autosomes [chomrosomes not associated with sex] + one X chromosome and one Y chromosome. In a "normal" woman the pattern is 44 autosomes + two X chromosomes. .
In the case of a man with Klinefelter's Syndrome, the defining genetic difference is that the person has at least two X chromosomes PLUS at least one Y chromosome. The chromosomal pattern is most often 44 autosomes + XXY. For simplicity, I'm going to write as if 44 +XXY is "the" Klinefelter's chromosomal description, but please keep in mind that there are variations, like 44 + XXYY. You'll notice that the sex chromosomes include both the XX that defines a female in most cases and the Y that causes male development. The X chromosomes are important, but for purposes of the embryo turning into a male, the Y is the important one. People with XXY are not hermaphrodites or intersexed - the Y chromosome(s) ensure that they will develop a penis instead of a clitoris, a scrotum instead of labia, and testes instead of ovaries (barring secondary abnormalities due to chomrosomal oddities or teratogenic exposures). .
In someone with KS, the cause of the XXY sex chromosomes was nondysjunction [failure to separate] of the X chromosomes, either in meiosis I of the father's spermatogenesis or else at either stage of meiosis (meiosis I or meiosis II) in the mother. When the egg & sperm met, they ended up with three (or more) sex chromosomes. The typical genetic pattern of a man with KS (44 +XXY) is found in about 1:500 males.
A note about nomenclature: Some of the newer sources refer to KS as "47,XXY." In these, they are counting the total number of autosomes + sex chromosomes to get the "47." Personally, I prefer the older "44+XXY" and believe it is clearer, so I've used that by preference. .
What the Label Means.
Klinefelter's is a genetic "syndrome.