This chromosomal abnormality in males, where there are two or more X chromosomes and one Y chromosome (47,XXY), is found with a frequency of 1 in 800 live male births.
These males grow tall and have disproportionally long arms and legs (thanks to howshealth.com for this image). They have narrow shoulders and a wide female looking pelvis. They enter puberty at the normal age, but facial hair growth is very light.
There is partial female breast development (gynecomastia). They have learning difficulties and their small testicles do not produce sperm. However, there is a wide variation and some males with Klinefelter syndrome have normal intelligence. Often this syndrome is only detected in infertility clinics. As testosterone production in the gonads is missing or extremely low, and there are high FSH levels in the blood because of the missing negative inhibition on the hypothalamus.
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