Mayo Clinic: Men’s Health Center


Karyotype is a blood test to identify and evaluate the size, shape and number of chromosomes a person has. Some men are born with an extra X chromosome (XXY), a condition called Klinefelter syndrome.  Klinefelter syndrome occurs as a result of a random error that causes a male to be born with an extra sex chromosome. It isn't an inherited condition.

Humans have 46 chromosomes, including two sex chromosomes that determine a person's sex. Females have two X sex chromosomes (XX). Males have an X and a Y sex chromosome (XY).

Klinefelter syndrome can be caused by:

  • One extra copy of the X chromosome in each cell (XXY), the most common cause
  • An extra X chromosome in some of the cells (mosaic Klinefelter syndrome), with fewer symptoms
  • More than one extra copy of the X chromosome, which is rare and results in a severe form

Extra copies of genes on the X chromosome can interfere with male sexual development and fertility.

Why it's done

A karyotype test may be recommended in men who are being seen for extremely low sperm counts (sperm concentration less than 5 million), or men with low testosterone and symptoms of Klinefelter syndrome. Signs and symptoms of Klinefelter syndrome may include:

  • Low sperm count or no sperm
  • Small testicles and penis
  • Low sex drive
  • Taller than average height
  • Weak bones
  • Decreased facial and body hair
  • Less muscular compared with other men
  • Enlarged breast tissue
  • Increased belly fat


A karyotype test generally has little physical risk. When the needle is inserted to draw blood, some people may feel some pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or a slight bruise.

A karyotype test can have emotional, social and financial risks as well. Discuss all risks and benefits of genetic testing with your doctor, a medical geneticist or a genetic counselor before you have a genetic test.

How to prepare

You do not need to be fasting when the labs are drawn, but you should discuss the implications of testing with a medical geneticist or genetic counselor prior to testing.


A normal karyotype test will have a total of 46 chromosomes including a pair of sex chromosomes. Men typically have and X and Y chromosome. Men with Klinefelter syndrome with have an copy of the X chromosome (XXY) in each cell, or an extra X chromosome in some of the cells (mosaic Klinefelter syndrome).

Did you know?

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