Tetrasomy X

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. Tetrasomy X is generally not inherited, but rather occurs via a random event called nondisjunction during gamete or zygote development. The formal term for the karyotype observed in tetrasomy X is 48,XXXX, as the condition is typified by a 48-chromosome complement rather than the 46 chromosomes observed in normal human development. Tetrasomy X has a variable presentation with a spectrum of severity, and lacks obvious defining clinical abnormalities that can lead to a diagnosis in the absence of testing. Recognizable characteristics include increased height and mild intellectual disability; the average adult height in tetrasomy X is compared with a reference height of around for women in the Anglosphere, while a review of the first 27 women to be diagnosed with tetrasomy X found IQs ranging from 30 to 101 with a mean of 62.