ABOUT 47,XXY (Klinefelter syndrome)

47,XXY is the most common of the X and Y variations, also known as “sex chromosome aneuploidy” (SCA). An estimated 1 in 500 males has an extra X chromosome.

One of the challenges of 47,XXY is that it is not characterized by any prominent physical features. Affected children and adults typically do not look much if any different than their peers. Subtle physical differences may include slightly taller stature, a curved little finger, small testes in adults, difficulty straightening out the elbows (“radio-ulner synostenosis”), flat feet, and a small depression in the chest.

47,XXY may produce the following additional symptoms, although most people with the condition will demonstrate only some, but not all, of them:

  • Low muscle tone
  • Infertility or lowered fertility
  • Hypogonadism (low levels of testosterone)
  • Speech delay/expressive language disorder
  • Mild autism
  • Subtle developmental delays
  • Learning disability
  • Attention deficits
  • Depression/anxiety


Klinefelter syndrome was first described in 1942 by Dr. Harry Klinefelter, an endocrinologist at Massachusetts General Hospital. He published a case study of nine adult men with a common set of symptoms involving hypogonadism. At the time, chromosomes had not yet been discovered. That occurred in 1956, when scientists confirmed that humans typically have 23 pairs of chromosomes, including two sex chromosomes. In 1959, a young English researcher, Patricia Jacobs, identified an extra X chromosome as the genetic marker for Klinefelter syndrome, and gave it the genetic signature of 47,XXY.
Because symptoms are subtle and vary widely from one person to the next, only 30 percent of affected individuals are diagnosed with Klinefelter syndrome during their lifetimes. Some may be identified prenatally due to maternal testing (via amniocentesis, CVS, or a non-invasive cell free DNA blood test). Another point at which 47,XXY, may be detected in an affected individual is during infertility testing in adulthood.

One of the goals of AXYS is to encourage families to be aware of the developmental, medical and psychosocial issues that may accompany 47,XXY. When children exhibit a history of symptoms such as speech delay, learning disability, mild autism, or delayed puberty, parents should follow up with genetic testing to determine whether the cause is an SCA.


Testing for 47,XXY involves a specialized blood test, either a “karyotyping” or a “microarray.” Health care professionals, including physicians, may be familiar with the endocrine features of Klinefelter syndrome, but not the developmental and cognitive difficulties associated with it. They often associate genetic syndromes with dysmorphia (characteristic facial or body features) or with intellectual disability, for which they fail to suggest genetic testing.

Myths About 47,XXY

Unfortunately, there are a number of myths about those affected by 47,XXY. Some of them associate it with criminality and pedophilia, the result of studies completed in the early 1960s within prison populations which have since been disproven as biased and unrepresentative of XXY adults. Other myths include assertions that all XXY men are gay or transgendered. While it is true that some XXY men identify as gay, and that some with XXY live as females, or prefer not to identify as either male or female, none of these is true for the majority of XXY individuals.

Frequently Asked Questions about 47,XXY

Downloadable Brochure: About 47,XXY (Klinefelter syndrome)

Living with Klinefelter Syndrome (47,XXY) Trisomy X (47, XXX) and 47, XYY: A Guide for Families and Individuals Affected by Extra X and Y Chromosome

Thanks to the generosity of the book’s author, Virginia Isaac’s Cover, MSW, AXYS is pleased to make this book available to the X and Y variation community, in its entirety, at no cost. Please feel free to download and/or print it for your personal use. (Those wishing to purchase a soft-bound copy can do so at Amazon or Kindle. All proceeds from the sale of this book benefit AXYS.)

“Spanish: A Guide for Families and Individuals Affected by X and Y Chromosome Variations”


Click here for an informative Medscape e-medicine article on Klinefelter syndrome.