| Boys normally have 46 chromosomes, including one X and one Y chromosome (46, XY). Girls also have 46 chromosomes, including two X chromosomes (46, XX).
Children with Klinefelter syndrome have one or more extra X chromosomes. It occurs in about 1 out of every 1000 newborn males, and most of these children will have a chromosome complement equal to 47, XXY, although other patterns also exist. Since you must have a Y chromosome to have this disorder, it affects only males.
The symptoms of Klinefelters may include a typical body appearance, being tall (taller than average compared to other family members) and usually underweight. They may also have long legs, gynecomastia, little facial hair, small testes for their age, and a smaller than average penis. Many of these symptoms are only noticeable during puberty.
Other symptoms can include mental retardation, learning disabilities, and behavioral problems, including being aggressive and immature. Children with Klinefelter syndrome will also usually have a delay in pubertal development and infertility.
It is important to keep in mind that not all children with an extra X chromosome will have all or any of these symptoms.
Diagnosis is made by doing a chromosome analysis, which will show the typical pattern of 47, XXY, or an alternative pattern, including mosaicism (having both normal and abnormal chromosomes), such as 46, XY/47, XXY or 46, XY/47, XXYY, or other variants with more than two X chromosomes. (48, XXXY).
Most children with Klinefelters are diagnosed at the time of puberty, when more of the symptoms are noticeable or because they fail to begin puberty. Others are diagnosed by prenatal testing.
Treatment is with hormone replacement with testosterone once the child is 11 to 12 years old.
Klinefelter Resource List:
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