XYY Syndrome is a genetic condition that occurs when a male’s genes have an extra Y chromosome. Females typically have two X chromosomes (XX). Males typically have one X chromosome and one Y chromosome (XY). Males with this genetic condition have an extra copy of the Y chromosome in each of their cells (XYY).
Most people have 46 chromosomes in each cell. Males with XYY syndrome have 47 because of the extra copy of the Y chromosome. However, sometimes this mutation is only present in some cells. According to the National Institutes of Health, XYY syndrome occurs in 1 out of every 1,000 boys (Genetics Home Reference).
XYY Syndrome is also sometimes called Jacob’s Syndrome, XYY Karyotype, or YY Syndrome.« Back to Glossary Index