Helpline 0300 111 4748

A brief review of the scientific background

Klinefelter’s Syndrome is a genetic condition that was named after Dr Harry Klinefelter, a Baltimore endocrinologist.

 In 1942, whilst working at the Massachusetts General Hospital, USA, he and his co-workers published a study about nine men who had enlarged breasts, sparse facial and body hair and small, firm, infertile testes.

By the late 1950s other researchers discovered that subjects with Klinefelter’s Syndrome (KS)/XXY had an extra sex chromosome. This happens by chance, usually during the formation of eggs or sperm.

Chromosomes are thread-like structures which carry genes and they are found in the nucleus of our body cells. With the exception of sperm and egg cells, each of our cells usually contain 46 chromosomes arranged in pairs. One of these pairs are the sex chromosomes. They are known as the X and Y chromosomes.

Eggs normally contain one X chromosome and sperm usually contain one X or one Y chromosome. Women generally inherit two X chromosomes – one from the mother’s egg and one from the father’s sperm. Men usually inherit an X chromosome from their mother and a Y chromosome from their father.


Occasionally, an egg with two X chromosomes, (XX), or a sperm with both an X and a Y chromosome, (XY) can occur. If an XY sperm fertilises an X egg or a Y sperm fertilises an XX egg the result will be the KS chromosome pattern (karyotype) 47, XXY instead of 46, XY. The 47 indicates that there are 47 chromosomes in each cell rather than the usual 46.

There are other less common variations – 48, XXXY and 49, XXXXY. Generally the greater the number of extra X chromosomes, the greater the effect. There is also a milder form of KS which occurs when only some of the body cells have the extra chromosome. This is known as a mosaic.  None of these are inheritable conditions.

Although KS/XXY was identified many years ago, there is little quality research. Many of the papers available were based on small, skewed samples, resulting in misleading conclusions.

Some XXY people believe that they do not suffer from Klinefelter’s Syndrome while others have been diagnosed with KS despite not having the extra chromosome.

For more information read KS or XXY? and VSC, DSD or intersex    

KS or XXY test