Genetics and 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 Y chromosome ( 46,XY ). Most often, Klinefelter syndrome results from the presence of one extra copy of the X chromosome in each cell ( 47,XXY ). Extra copies of genes on the X chromosome interfere with male sexual development, often preventing the testes from functioning normally and reducing the levels of testosterone. Most people with an extra X chromosome have the features described above, although some have few or no associated signs and symptoms.

Some people with features of Klinefelter syndrome have more than one extra sex chromosome in each cell (for example, 48,XXXY or 49,XXXXY). These conditions, which are often called variants of Klinefelter syndrome , tend to cause more severe signs and symptoms than classic Klinefelter syndrome . In addition to affecting male sexual development, variants of Klinefelter syndrome are associated with intellectual disability, distinctive facial features, skeletal abnormalities, poor coordination, and severe problems with speech. As the number of extra sex chromosomes increases, so does the risk of these health problems.

Some people with features of Klinefelter syndrome have the extra X chromosome in only some of their cells; in these individuals, the condition is described as mosaic Klinefelter syndrome (46,XY/47,XXY). Individuals with mosaic Klinefelter syndrome may have milder signs and symptoms, depending on how many cells have an additional X chromosome .

Source: GHR (NLM/NIH)1

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Back to: « Klinefelter Syndrome

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Inheritance Pattern

Klinefelter syndrome and its variants are not inherited; these chromosomal changes usually occur as random events during the formation of reproductive cells (eggs and sperm) in a parent. 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.

Mosaic 46,XY/47,XXY is also not inherited. It occurs as a random event during cell division early in fetal development. As a result, some of the body's cells have one X chromosome and one Y chromosome (46,XY), and other cells have an extra copy of the X chromosome (47,XXY).

Source: GHR (NLM/NIH)2

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Learn more about the chromosome associated with Klinefelter syndrome

  • X chromosome

Source: GHR (NLM/NIH)3

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Inheritance

Klinefelter syndrome is not inherited, but usually occurs as a random event during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction can result in reproductive cells 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 reproductive cells contributes to the genetic makeup of a child, the child will have one or several extra X chromosomes in each of the body's cells.[2] Last updated: 10/12/2015

Source: GARD (NIH)4

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Usually, a female baby has two X chromosomes (XX) and a male has one X and one Y (XY). But in Klinefelter syndrome, a boy is born with an extra copy of the X chromosome (XXY).

The X chromosome is not a "female" chromosome and is present in everyone. The presence of a Y chromosome denotes male sex.

Boys and men with Klinefelter syndrome are still genetically male, and often will not realise they have this extra chromosome, but occasionally it can cause problems that may require treatment.

Source: NHS Choices UK5

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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.

Source: NHS Choices UK6

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References

  1. Source: GHR (NLM/NIH): ghr.nlm.nih.gov/ condition/ klinefelter-syndrome
  2. ibid.
  3. ibid.
  4. Source: GARD (NIH): rarediseases.info.nih.gov/ diseases/ 8705/ klinefelter-syndrome
  5. Source: NHS Choices UK: nhs.uk/ conditions/ klinefelters-syndrome/ 
  6. ibid.

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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.