Klinefelter Syndrome Causes

Klinefelter syndrome is a condition that occurs as a result of an extra X chromosome during the formation of either a sperm or an egg cell. It is usually a random error that causes the extra X cell to be included in each cell division. There can sometimes be more than one additional X chromosomes, although this is rare.

Although Klinefelter syndrome is a genetic condition, it is not hereditary or inherited from the parents. Instead, it is believed to occur due to a random error during the cell division of the sperm or egg cell that is responsible for giving rise to the the fetus.

Physiology of Sex Chromosomes

Humans usually have 46 pairs of chromosomes. Two of the 46 chromosomes determine the sex of the individual and are thus called sex chromosomes.  The remaining 44 chromosome determine other factors. The mother is responsible for passing on one X chromosome to her child and the father may provide another X chromosome for a daughter or a Y chromosome for a son.

A boy born with Klinefelter syndrome has an extra X chromosome that is believed to be caused by a random error. This error usually occurs during meiosis when the cell divides to produce the sperm or the egg cell in the father or mother, respectively.

As males with Klinefelter syndrome have one Y chromosome, they will have some male sexual characteristics. However, the additional X chromosome can hinder the development of the testicles and the production of the male hormone testosterone, leading to symptoms of the condition.

Variants of Klinefelter Syndrome

In rare cases of Klinefelter syndrome, multiple extra X chromosomes may be involved. The likelihood of the variants of Klinefelter syndrome in newborn males is estimated to be as follows:

  • One extra X chromosome (XXY): 1 in 500 newborn males
  • Two extra X chromosomes (XXXY): 1 in 50,000 newborn males
  • Three extra X chromosomes (XXXXY): 1 in 85,000-100,000 newborn males

These variants of Klinefelter syndrome are rarer as they involve additional random errors. This may occur because there is an error in both the sperm and egg cell of the fetus or additional errors in one of these cells.

Risk Factors

The error that leads to the production of the extra X chromosome appears to be random and is not hereditary (i.e. not inherited from the parents).

Newborn males with a mother who is older may have a slightly increased risk of the syndrome in comparison to other newborns of younger mothers. However, almost 50% of cases of Klinefelter syndrome are thought to originate from an error in the X chromosome of the sperm from the father and are, therefore, not linked to the age of the mother. Older age of the father may also be linked to an increased risk of the syndrome.

Other risk factors associated with Klinefelter syndrome are not well known and research is currently being undertaken to investigate this further. A deeper understanding of Klinefelter syndrome and the causes of the condition could potentially help in its prevention and treatment.


  • https://www.nichd.nih.gov/health/topics/klinefelter/conditioninfo/Pages/risk.aspx
  • https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/klinefelter-syndrome
  • http://emedicine.medscape.com/article/945649-clinical#b5
  • https://ghr.nlm.nih.gov/condition/klinefelter-syndrome#inheritance

Further Reading

  • All Klinefelter Syndrome Content
  • Klinefelter Syndrome
  • Klinefelter Syndrome Diagnosis
  • Klinefelter Syndrome Treatment
  • Klinefelter Syndrome Signs and Symptoms

Last Updated: Feb 26, 2019

Written by

Yolanda Smith

Yolanda graduated with a Bachelor of Pharmacy at the University of South Australia and has experience working in both Australia and Italy. She is passionate about how medicine, diet and lifestyle affect our health and enjoys helping people understand this. In her spare time she loves to explore the world and learn about new cultures and languages.

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