Klinefelter syndrome is a disorder of sex development (DSD) in which a child who is genetically male (or whose sex assigned at birth is male) is born with an extra X chromosome.
When an embryo forms, its biological sex develops depending on chromosomes received from each parent. The mother contributes an X chromosome, while the father contributes either an X or Y chromosome. The Y chromosome contains the genetic data for male development, so male babies typically have an XY chromosome combination. (Females have an XX combination.)
In Klinefelter syndrome, the fetus receives an additional X chromosome. As a result, the baby’s chromosome makeup is XXY. The baby is considered to be biologically male, but the extra X chromosome affects the full development of male characteristics.
For most men with Klinefelter syndrome, the extra X chromosome is found in all cells. If the extra chromosome is present in only some of the cells, the boy is said to have mosaic Klinefelter syndrome. In rare cases, more than one extra X chromosome is found.
Experts estimate that Klinefelter syndrome affects about 1 in 650 males.
Because of the extra X chromosome, individuals with Klinefelter syndrome experience puberty differently from the way their peers do. For some, puberty is delayed or doesn’t occur.
For others, secondary sex characteristics don’t fully develop. For example, young adolescents with Klinefelter syndrome may have less facial hair than their peers. Their testicles and penis may be smaller than average. Some develop breasts (gynecomastia) and have higher-pitched voices.
As adults, individuals with Klinefelter syndrome typically have lower levels of testosterone. As a result, their sex drive might be diminished, and they could develop erectile dysfunction. They might also have less muscle mass and less energy than their peers or other individuals with penises.
These symptoms can often be treated with testosterone replacement therapy, with the guidance of a healthcare provider. Some individuals with Klinefelter syndrome start taking testosterone at puberty.
Infertility is another common issue for men with Klinefelter syndrome. Many do not produce sperm, and if they do, the amount is small.
However, this does not mean that men with Klinefelter syndrome cannot start families. If sperm are present in the testicles, specialists may be able to remove them with a biopsy needle. The sperm can then be injected into eggs.
Couples may also consider using sperm from a donor or adopting a child.
If you have Klinefelter syndrome, be sure to see your healthcare provider regularly.
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