| XXXYY syndrome | |
|---|---|
| Other names | 49,XXXYY |
 | |
| Karyotype of XXXYY syndrome | |
| Specialty | Medical genetics  |
| Symptoms | Intellectual disability, facial dysmorphisms, genital anomalies |
| Usual onset | Conception |
| Duration | Lifelong |
| Causes | Nondisjunction |
| Diagnostic method | Karyotype |
XXXYY syndrome, also known as 49,XXXYY, is a chromosomal disorder in which a male has three copies of the X chromosome and two copies of the Y chromosome. XXXYY syndrome is exceptionally rare, with only eight recorded cases. Little is known about its presentation, but associated characteristics include intellectual disability, anomalies of the external genitalia, and characteristic physical and facial features. It is not caused by characteristics of the parents, but rather occurs via nondisjunction, a random event in gamete development. The karyotype observed in the syndrome is formally known as 49,XXXYY, which represents the 49 chromosomes observed in the disorder as compared to the 46 in normal human development.
XXXYY syndrome was first recorded in 1963. Its long-term prognosis is poorly understood; while the condition as reported in the medical literature is relatively severe, it is unknown if there are milder cases that have not come to diagnostic attention.