Tetrasomy X, also known as 48,XXXX, is a chromosomal disorder in which a female has four, rather than two, copies of the X chromosome. It is associated with intellectual disability of varying severity, characteristic "coarse" facial features, heart defects, and skeletal anomalies such as increased height, clinodactyly (incurved pinky fingers), and radioulnar synostosis (fusion of the long bones in the forearm). Tetrasomy X is a rare condition, with few medically recognized cases; it is estimated to occur in approximately 1 in 50,000 females.
The disorder has a wide range of symptoms, with phenotypes (presentations) ranging from slight to severe. It is suspected to be underdiagnosed, as are other sex chromosome disorders. Life outcomes vary; some women have had education, employment, and children, while others have remained dependent into adulthood. Life expectancy does not appear to be substantially reduced. Tetrasomy X has phenotypic overlap with a number of more common disorders, such as trisomy X and Down syndrome, and diagnosis is usually unclear prior to chromosomal testing.
