Causes of Klinefelter Syndrome

What causes Klinefelter syndrome (KS)?

The extra chromosome results from a random error that occurs when a sperm or egg is formed; this error causes an extra X cell to be included each time the cell divides to form new cells. In very rare cases, more than one extra X or an extra Y is included.

Source: NICHD (NIH)1

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Back to: « Klinefelter Syndrome

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Klinefelter syndrome usually occurs as a random event during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction results in a reproductive cell with an abnormal number of chromosomes. For example, an egg or sperm cell may gain one or more extra copies of the X chromosome as a result of nondisjunction. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have one or more extra X chromosomes in each of the body's cells.[2]

Most often, Klinefelter syndrome is caused by a single extra copy of the X chromosome, resulting in a total of 47 chromosomes per cell. Males normally have one X chromosome and one Y chromosome in each cell (46, XY), while females have two X chromosomes (46, XX). People with Klinefelter syndrome usually have two X chromosomes and one Y chromosome (47, XXY). Some people with Klinefelter syndrome have the extra X chromosome in only some of their cells; these people are said to have mosaic Klinefelter syndrome.[2]

It is estimated that about half of the time, the cell division error occurs during development of the sperm, while the remainder are due to errors in egg development. Women who have pregnancies after age 35 have a slightly increased chance of having offspring with this syndrome.[4]

The features of Klinefelter syndrome are due to the extra copies of genes on the extra X chromosome, which can alter male sexual development.[3] Last updated: 10/12/2015

Source: GARD (NIH)2

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Causes of Klinefelter syndrome

Klinefelter syndrome is caused by an additional X chromosome.

This chromosome carries extra copies of genes, which interfere with the development of the testicles and mean they produce less testosterone (male sex hormone) than usual.

The extra genetic information may either be carried in every cell in the body or it may only affect some cells (known as mosaic Klinefelter syndrome).

Klinefelter syndrome isn't directly inherited - the additional X chromosome occurs as a result of either the mother's egg or the father's sperm having the extra X chromosome (an equal chance of this happening in either), so after conception the chromosome pattern is XXY rather than XY.

This change in the egg or sperm seems to happen randomly. If you have a son with the condition, the chances of this happening again are very small.

However, the risk of a woman having a son with Klinefelter syndrome may be slightly higher if the mother is over 35 years of age.

Source: NHS Choices UK3

Causes List for Klinefelter Syndrome

Some of the possible causes of Klinefelter Syndrome or similar disorders may include:4

... Full Causes List for Klinefelter Syndrome »

Genetics of Klinefelter Syndrome

Genetic Changes: Klinefelter syndrome is a condition related to the X and Y chromosomes (the sex chromosomes ). People typically have two sex chromosomes in each cell: females have two X chromosomes (46,XX), and males have one X and one ...5

... More on Genetics »

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  1. Source: NICHD (NIH): health/ topics/ klinefelter/ conditioninfo/ Pages/ Default.aspx
  2. Source: GARD (NIH): diseases/ 8705/ klinefelter-syndrome
  3. Source: NHS Choices UK: conditions/ klinefelters-syndrome/ 
  4. Source: Algorithmically Generated List
  5. Source: GHR (NLM/NIH): condition/ klinefelter-syndrome

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Note: This site is for informational purposes only and is not medical advice. See your doctor or other qualified medical professional for all your medical needs.