Question: What is (are) Tetrasomy X ?
Answer: Tetrasomy X is a chromosome disorder that only affects females and is caused by having four copies of the X chromosome instead of two. Females with tetrasomy X have a total of 48 chromosomes in their cells, so this condition is sometimes written as 48, XXXX. The signs and symptoms of tetrasomy X vary, but can include mild to moderate speech and learning difficulties; developmental delay; distinctive facial features; dental abnormalities; hypotonia and joint laxity; radioulnar synostosis; heart defects; hip dysplasia; and problems with ovarian function. An increased risk of childhood infections has also been reported. Tetrasomy X is caused by a random error that occurs during the development of an egg cell and is not caused by anything a mother does during her pregnancy.
Source: GARD

Question: What causes Tetrasomy X ?
Answer: What causes tetrasomy X? Tetrasomy X is usually caused by a random error in the formation of an egg cell (before pregnancy). In some cases, it may be due to inheriting three X chromosomes from the mother and one X chromosome from the father. In other cases, it may be due to inheriting all four X chromosomes from the mother. During the normal formation of egg cells, each egg cell contains one X chromosome to pass on to offspring. However, errors in cell division can cause an egg cell to have three or four X chromosomes, instead of one. If an egg cell with the extra X chromosomes is fertilized by a sperm cell with one X chromosome, the resulting embryo will have these extra chromosomes. Rarely, tetrasomy X may be caused by an error in cell division that occurs after an egg is fertilized, or by the presence of extra X chromosomes in some of the mother's cells.
Source: GARD