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Klinefelter Syndrome

Klinefelter Syndrome

Alternate Names

  • XXY syndrome


Klinefelter syndrome is a genetic disorder that occurs in males who have three or more sex chromosomes. At least two of the chromosomes must be X chromosomes, and one must be a Y chromosome. The condition causes males to have some female-like physical features.

What is going on in the body?

Klinefelter syndrome affects only males. The presence of the Y chromosome assures that male sexual features will be present.
Because males with Klinefelter syndrome don't have the normal number of chromosomes, the body's natural balance is upset. The upset in balance leads to:
  • underdevelopment of the testicles, a condition called hypogonadism, which usually leads to infertility
  • a taller-than-average height and a feminine body build
Klinefelter syndrome usually does not cause mental retardation, but affected males test slightly lower than their brothers and sisters on IQ testing. They also may have problems talking and hearing.


What are the causes and risks of the condition?

Klinefelter syndrome is caused when a fertilized egg has an extra sex chromosome.
Most people have 46 chromosomes in the nucleus of each cell of their bodies. These chromosomes exist in matched pairs, meaning there are 23 pairs. One of these 23 pairs determines whether a person will be a male or a female. This pair is called the sex chromosome pair.
The sex chromosomes are designated by letters. The letter X designates the chromosome associated with being female. The letter Y designates the chromosome associated with being male.
Normal females have two X chromosomes. Normal males have an X chromosome and a Y chromosome.
If two X chromosomes are present, and there is also a Y chromosome, the person will be a male with the Klinefelter syndrome. This condition is also called 47,XXY.
Some people with Klinefelter syndrome have more two X chromosomes and more than one Y chromosome.
The physical features of Klinefelter syndrome are caused by abnormally low production of the male sex hormones. Klinefelter syndrome is seen in 1 in 1,000 total live births, or 1 in 500 male live births.


What can be done to prevent the condition?

There is no way to prevent this condition. Genetic counseling is useful for affected people and their families.


How is the condition diagnosed?

Klinefelter syndrome usually is not obvious at birth, unless some of uncommon physical features are present. It may be suspected at puberty, when secondary sexual characteristics do not develop. The penis usually enlarges normally, but the testicles remain small and beard growth is sparse. In addition, breasts may enlarge at puberty. While measuring certain hormones is useful, chromosome analysis is the definitive diagnostic test.

Long Term Effects

What are the long-term effects of the condition?

Infertility is the most obvious long-term effect of Klinefelter syndrome. Many affected individuals, however, develop diabetes or thyroid disease. Life span is usually normal.

Other Risks

What are the risks to others?

Klinefelter syndrome is not contagious. Since males with Klinefelter syndrome are usually infertile, there is little risk they will pass the disorder on to children.


What are the treatments for the condition?

Testosterone is used to treat Klinefelter syndrome. Treatment must be timed carefully to coincide with the onset of puberty. If a learning disability is present, special education may be needed. Psychotherapy may be needed if emotional maturity and adjustment are problems.

Side Effects

What are the side effects of the treatments?

If the timing of treatment is not balanced carefully, excessive growth may occur.

After Treatment

What happens after treatment for the condition?

Treatment to maintain secondary sexual characteristics must continue throughout life.


How is the condition monitored?

Klinefelter syndrome itself is not progressive and does not need to be monitored. Effects of treatment for symptoms are monitored by growth measurements, x-rays, and hormone levels.


Jones KL: Recognizable Patterns of Human Malformation. WB Saunders, 1997.

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