Klinefelter syndrome is a genetic condition in which a male has an extra X sex chromosome. Chromosomes are the carriers of inheritance, and sex chromosomes help determine an individual’s sex. Human beings normally have two sex chromosomes, with females having two X chromosomes (typically written XX) and males having one X and one Y chromosome (XY). Males with Klinefelter syndrome have two X chromosomes and one Y (XXY). The symptoms of Klinefelter syndrome can vary greatly and may go unnoticed until affected boys reach puberty. At that time, they typically experience incomplete sexual development. Klinefelter syndrome occurs in about 1 in 500 male births. The syndrome is named after Harry Klinefelter, the American physician who, with his colleagues, first described it in a medical journal in 1942.
Klinefelter syndrome is an example of aneuploidy—the condition of having an abnormal number of chromosomes. Each human body cell, except for sperm and egg cells, typically has 46 chromosomes, consisting of 23 similar pairs. A fertilized embryo typically includes one member of each pair—including one X chromosome—from the mother’s egg cell and one member—including either an X or Y sex chromosome—from the father’s sperm. Males with Klinefelter syndrome have 47 chromosomes. They inherit an extra X chromosome when chromosome pairs fail to separate properly during the formation of sperm or egg, a defect called nondisjunction.
Medical experts diagnose Klinefelter syndrome by organizing images of the individual’s chromosomes into a profile called a karyotype. A karyotype can be prepared before birth through a procedure called amniocentesis (see Amniocentesis ).
Boys with Klinefelter syndrome often have weak muscles, so they may crawl and walk at a later age than other babies. They are often taller and less muscular than other boys, with broader hips and a distribution of body fat similar to that of females. At puberty, men with Klinefelter syndrome usually experience incomplete development of secondary sexual characteristics, physical traits such as body shape, voice, and patterns of hair growth associated with the onset of sexual maturity. They also exhibit low levels of testosterone, a hormone that stimulates sexual development in males. They usually have sparse body hair and small testes and may experience growth of breast tissue, a condition called gynecomastia.
Boys with Klinefelter syndrome usually exhibit an intelligence within the normal range but may experience learning disabilities, particularly in language and verbal skills. They frequently have problems with reading, writing, and spelling. They are often shy and less confident than other children in social interactions. Men with Klinefelter syndrome may have less interest in sex due to low testosterone. They are typically infertile.
Variations of Klinefelter syndrome do occur. Some males have 3 or even more X chromosomes due to nondisjunction. Such cases are less common, and the symptoms are typically similar to Klinefelter syndrome but more severe. Males with mosaic Klinefelter syndrome have some cells of their body that contain an extra X chromosome while other cells are normal. This condition arises when the nondisjunction occurs after a fertilized egg has begun dividing to form an embryo. The symptoms of mosaic Klinefelter syndrome are often less severe than other types.
Treatment for Klinefelter syndrome includes testosterone replacement therapy during puberty to help increase muscle strength and the growth of body hair. Boys with Klinefelter syndrome may also take special classes to improve language skills and self-esteem. They and their families often benefit from participation in support groups and from professional counseling (see Genetic counseling ).
See also Aneuploidy .
