In normal male development, there is one X chromosome and one Y chromosome. But in the rare disorder called Klinefelter syndrome, patients have an extra X chromosome or part of one in most cells of the body (which is why the disorder is also called XXY condition).
Persons with Klinefelter syndrome produce less testosterone and may have more feminized physical features such as smaller testicles and less facial and body hair than other males, as well as breast enlargement. Persons may also suffer from infertility. (Ironically, though, some children and adults with the disorder may be taller than their counterparts of the same age, according to a Klinefelter syndrome article from the Genetics Home Reference.)
Unfortunately for Klinefelter sufferers, the disorder has more than just these physical symptoms. Boys with Klinefelter syndrome may also have problems with speech and language development, and may have learning disabilities.
You can find more information on Klinefelter syndrome at the government's Medline online medical encyclopedia, which gives a good lay overview.
For details on clinical research being done on Klinefelter syndrome, the National Human Genome Research Institute has an overview with links to research studies and additional resources.
