Tony                                                    rare 2n = 48,XXYY form

Klinefelter Syndrome (2n = 47, XXY)

    Klinefelter Syndrome is the most common chromosomal syndrome, and occurs in about 1 in 500 live births. The basic male phenotype is set by the Y chromosome: duplication of the X chromosome genes often contributes to a hormonal makeup (hypogonadism) intermediate between XX females and XY males. XXY individuals often have a low serum testosterone level, high serum follicle-stimulating hormone (FSH), and luteinizing hormone (LH) levels. XXY men often have microorchidism (small testicles) and are typically infertilePhysical characteristics vary widely, from a tall, thin, youthful or somewhat feminized physique and facial appearance, to a shorter, rounded body type. Secondary sex-characteristics such as pubic hair patterns are often feminized, and there is often some degree of gynecomastia (female-type breast development), with a significantly higher risk of breast cancer. As with Turner Syndrome (XO females), there is a range of intellectual developmental, with specific neuro-psychological impairments including language ability and deficits in executive functions. Social stigmatization is a problem. There is apparently no increased tendency to homosexuality.

    As with other chromosomal syndromes, the standard 'syndrome' may be due in part to ascertainment bias: XXY males are often diagnosed as such when they present with overt physical characteristics, whereas other XXY individuals are asymptomatic. Tony [Left] worked as a computer programmer, married, and was identified as XXY only when the couple sought infertility counselling. He shows an atypical baldness pattern often associated with XYY. The man on the right, with a rarer XXYY karyotype, shows uncorrected gynecomastia.

Figure ©2002 by Griffiths et al.; all text material ©2010 by Steven M. Carr