What is Klinefelter syndrome?
Klinefelter syndrome is a genetic disease that causes biological males to be born with an extra copy of the X chromosome in their cells. Males with this condition may not have any symptoms or may have a variety of symptoms, including smaller-than-normal testicles that produce less testosterone.
Testosterone is the male hormone that stimulates sexual traits like body hair and muscle growth.
A lack of testosterone can cause symptoms, such as:
- breast growth
- a smaller penis
- less facial and body hair than typical
How common is it?
Klinefelter syndrome is one of the most common chromosome conditions in newborns. It affects between 1 in 500 and 1 in 1,000 newborn males. Variants of the disease with three, four, or more extra X chromosomes are less common. These variants affect 1 in 50,000.
It’s possible that Klinefelter syndrome affects even more male children and adults than statistics suggest. Sometimes symptoms are so mild that they go unnoticed. Or symptoms can be misdiagnosed as being caused by other similar conditions.
What causes Klinefelter syndrome?
Everyone is born with 23 pairs of chromosomes, or 46 chromosomes in total, inside each of their cells. These include two sex chromosomes, X and Y.
- People who are born female have two X chromosomes, XX. These chromosomes give them sexual traits like breasts and a uterus.
- People who are born male have one X and one Y chromosome, XY. These chromosomes give them traits such as a penis and testicles.
- People with Klinefelter syndrome are born with an extra X, causing their cells to have XXY chromosomes. This happens randomly during conception. About half the time the extra chromosome starts in the mother’s egg. The other half of the time it comes from the father’s sperm.
How is it diagnosed?
A small number of males with Klinefelter syndrome receive a diagnosis before birth when their mother has one of these tests:
Amniocentesis
. During an amniocentesis, a technician removes a small amount of amniotic fluid from the sac surrounding the baby. The fluid is then examined in a lab for chromosome problems.
Chorionic villus sampling
. Cells from tiny finger-like projections called chorionic villi in the placenta are removed during a chorionic villus sampling. These cells are then tested for chromosome problems.
Because these tests can increase the risk of a miscarriage, they usually aren’t done unless the baby is at risk for a chromosome problem. Often, Klinefelter syndrome isn’t discovered until a child reaches puberty or later.
