The cells of a normal, healthy person contain two copies of each of 23 chromosomes, for a total of 46 chromosomes per cell. Everyone has two sex chromosomes. Males have one X and one Y chromosome, while females have two X chromosomes. The X and Y chromosomes are called sex chromosomes because they contain the genetic information that determines male and female sexual characteristics. Klinefelter syndrome occurs when a male is born with an extra X chromosome in all or most of his cells. Sometimes people with this disorder are called XXY males. Even though a person with Klinefelter syndrome has an extra X chromosome, he is considered a male because he has a Y chromosome.
The number of cells with an extra X chromosome varies among patients. When only some of the person's cells have an extra X chromosome, the condition is called mosaic Klinefelter syndrome. The severity of symptoms increases with the number of cells that have an extra chromosome. There are also several variants of Klinefelter syndrome, including 48 XXYY syndrome; 48 XXXY syndrome; 49 XXXXY syndrome; and XXXY syndrome.
The most common symptom of Klinefelter syndrome is infertility because the genetic mutation causes males to produce less of the male hormone testosterone than healthy adult men. When the sex organs are underactive, it is called hypogonadism. Other symptoms may include reduced body and muscle tone (compared to males of the same age), enlarged breasts (called gynecomastia), difficulty communicating with others or expressing themselves, shyness, and intellectual and learning disabilities. In addition, males with this disorder have an increased risk of developing breast cancer and osteoporosis.
It is estimated that Klinefelter syndrome affects about one out of 500-1,000 newborn males. The additional X chromosome occurs because of a random error during the development of sperm or egg cells. Mosaic Klinefelter syndrome is caused by a random mutation during fetal development. The risk for having a child with Klinefelter syndrome increases as the mother ages. Mothers who are 35 and older have the greatest risk of having children with this disorder.
Most men with Klinefelter syndrome are able to live long, healthy lives. Although there is no cure for the disorder, treatment can effectively improve symptoms. Treatment with testosterone therapy, educational services, physical therapy, speech therapy, and/or occupational therapy helps most patients have normal sex lives, successful careers, and normal social relationships. Some males are able to have children, but they usually need the help of fertility treatments.
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