rare 2n = 48,XXYY
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 infertile. Physical 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
the couple sought infertility counselling. He shows an atypical
baldness pattern often associated with XYY.
man on the right, with a rarer XXYY
karyotype, shows uncorrected gynecomastia.
Griffiths et al.; all text
©2010 by Steven M. Carr