Genetics Northern California

XYY Syndrome

Boys with XYY have an extra Y chromosome. Instead of having one X and one Y sex chromosome, they have one X and two Y chromosomes. This occurs in approximately one in every 1000 male births, which means that one boy is born with this syndrome almost every day in California.

Most often, this chromosomal change causes no unusual physical features or medical problems. Males with 47,XYY syndrome are sometimes taller than average and have an increased risk of learning disabilities and delayed speech and language skills. Developmental delays and behavioral problems are also possible, but these characteristics vary widely among affected boys and men. Most males with 47,XYY syndrome have normal sexual development and are able to conceive children.

47,XYY syndrome is caused by an extra Y chromosome in each cell of the body. This happens completely by chance in pregnancies of women from all ages and ethnic backgrounds. Sex chromosome abnormalities are some of the most common chromosome abnormalities. Approximately 2 to 3 babies are born with a sex chromosome abnormality every day in California.

A chromosome study, either by prenatal testing or by blood test after birth, can accurately diagnose 47,XYY.

In most cases, the diagnosis of 47,XYY is never recognized because there are usually no specific medical findings that lead to testing. About 5% of boys with XYY are identified through prenatal diagnosis.  If prenatal diagnosis is not done, a small percentage of boys may be diagnosed because of learning problems, but about 88% of males with this condition live their entire lives unaware of their diagnosis.

Your chromosomes are in almost every cell of your body and it is impossible to correct the number of sex chromosomes present. However, if a boy with 47,XYY has problems with learning or development, services are available. Treatment of learning disabilities is the same for any child with similar motor, reading, or language delays. With early intervention and family support, most children with XYY have performed just as well as their chromosomally normal peers.


Genetics Home Reference: XYY syndrome - General information about XYY syndrome provided by the National Library of Health. Also includes links to related sites.