47,XXY: The Klinefelter Syndrome Karyotype Explained
Last updated: October 22, 2026
47,XXY
47,XXY is the karyotype of a male with an extra X chromosome, the most common cause of Klinefelter syndrome. Here is what the notation means, how to confirm two X chromosomes and a Y at the microscope, and the variant karyotypes to know.
What 47,XXY means
| Part | Meaning |
|---|---|
| 47 | 47 chromosomes in each cell analyzed, one more than the usual 46 |
| XXY | Two X chromosomes and one Y: the extra chromosome is an X |
Because the extra chromosome is a sex chromosome, it is shown in the sex chromosome part of the karyotype and needs no plus sign. Writing 47,XY,+X is incorrect.
What to look for in the karyotype
- A count of 47.
- 16 C-group chromosomes instead of the 15 a normal male has: the seven pairs 6 to 12 plus two X chromosomes.
- A Y chromosome in the G group, with no satellites and a variable long arm.
- Telling the X from chromosome 7. The X has a dark band in the middle of the short arm and another at a matching position on the long arm, giving an almost mirror-image look. Chromosome 7 has a dark cap at the top of the short arm and two prominent dark bands on the long arm.
Count enough cells. Some men have a mosaic karyotype with a normal 46,XY cell line alongside the 47,XXY line, and the proportion of each varies between tissues.
How 47,XXY happens
47,XXY results from nondisjunction of the sex chromosomes in the formation of an egg or sperm, or less often in the early embryo. The extra X comes from the mother or the father in roughly equal proportions. The maternal share is linked to older maternal age, as with other trisomies. It is almost always a new event, not inherited.
Related karyotypes
| Karyotype | Meaning |
|---|---|
| mos 47,XXY[18]/46,XY[12] | Mosaic Klinefelter syndrome, often with milder features |
| 48,XXXY | Two extra X chromosomes, with more marked features |
| 48,XXYY | An extra X and an extra Y, now usually considered a separate condition |
| 47,XYY | A different condition: an extra Y chromosome |
| 47,XXX | The female counterpart: trisomy X |
About Klinefelter syndrome
Klinefelter syndrome is one of the most common chromosome conditions, affecting roughly 1 in 500 to 1,000 males, yet many men are never diagnosed. Typical features include small testes, low testosterone, infertility, tall stature, and in some cases enlarged breast tissue. Some boys have language or learning difficulties. Testosterone therapy and fertility treatment can help many affected men.
Common questions
Why isn't 47,XXY written with a plus sign?
In ISCN, sex chromosome gains and losses are shown by listing the sex chromosomes actually present. Plus and minus signs are used for autosomes, as in 47,XY,+21.
Is 47,XXY inherited?
No. It is almost always a new event from nondisjunction in one parent's egg or sperm.
Keep learning
This page is for education and training. It is not medical advice and must not be used to diagnose or treat anyone. Laboratory results should be interpreted by qualified professionals following current standards.