Dictionary of Human Evolution and Biology

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Klinefelter Syndrome

Symptoms of this aneuploid condition (47, XXY) appear at puberty: affected males are tall with long limbs, experience a precocious puberty but lack secondary sexual characters, have hypogonadism (small penis and prostate gland), breast swelling (gynecomastia), a protruding stomach, taurodont dentition, and irreversible sterility caused by primary testicular failure. Osteoporosis is common in later adulthood. Cause is due to the dosage effect of a supernumerary X chromosome, and one Barr body is present. Androgen therapy and subcutaneous mastectomy restores a male phenotype in milder cases. A tendency toward learning disabilities increases in proportion to the number of supernumerary X chromosomes. Affects 1: 500 males; up to 1% of clinically institutionalized males are Klinefelter patients.

See XX male syndrome and primary nondisjunction.

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