Most girls and women have two X sex chromosomes (so we say their chromosomal component is “46,XX”). Most boys and men have an X sex chromosome and a Y sex chromosome (“46,XY”). Klinefelter syndrome occurs when a person is born with at least one Y chromosome and multiple X chromosomes. So the typical case of Klinefelter syndrome involves 47,XXY.
Klinefelter syndrome is fairly common; it is estimated to occur once in every 500 males. Boys and men born with Klinefelter syndrome typically will have smaller-than-average testicles and low fertility. (Various technologies can be used to promote assisted reproduction.) There are some other differences associated with Klinefelter syndrome, including some learning and behavioral differences, but the effects from Klinefelter syndrome vary substantially. Some affected boys and men show very few signs of having Klinefelter syndrome, while others show more signs, including gynecomastia (more female-type breast development) following puberty.
Posted in: Specific Conditions