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.^[1]

Because there are few reports in the medical literature about people with tetrasomy X, there is limited information about the recurrence risk for this condition (i.e. the risk to have a second affected child). Much of the information available is based on women with a previous pregnancy affected by a trisomy (3 copies of a chromosome), which is much more common than a tetrasomy. However, the recurrence risk for numerical sex chromosome abnormalities in general (extra or missing X or Y chromosomes) is thought to be very low.^[2]

The main factor known to increase a person's risk to have a baby with an abnormal number of chromosomes is maternal age. Women are born with all the eggs they will ever have. As a woman ages, so do her eggs. Errors may develop in the genetic material of eggs over time. Because of this, as a woman's age increases, the risk to have a baby with a chromosome abnormality increases.^[3]

People with specific questions about recurrence risks for themselves or family members should speak with a genetics professional.

Yes. Prenatal testing can be performed during pregnancy to determine if a fetus has a chromosome abnormality. There are two procedures most commonly done for this purpose. One is called chorionic villus sampling (CVS), typically done in the first trimester (between weeks 11 and 13). The other procedure is called amniocentesis, typically done in the second trimester or later (from about week 16 onward). Genetic counseling is recommended for people who are interested in prenatal testing for chromosome abnormalities.

Most sex chromosome abnormalities, including tetrasomy X, are thought to be due to a random error in the formation of egg or sperm cells; this type of error cannot be prevented. However, prenatal diagnosis via chorionic villus sampling or amniocentesis is available to test for chromosome abnormalities during the pregnancy.

If a person is known to have a very increased risk to have a child with a specific genetic abnormality, preimplantation genetic diagnosis (PGD) may be available. To have PGD, a couple must first undergo in vitro fertilization (IVF). Each embryo is tested for the condition, and only the embryos that are not affected are then transferred into the mother.

People interested in having preimplantation genetic diagnosis should speak with their health care provider or a genetics professional.

To find a medical professional who specializes in genetics, you can ask your doctor for a referral or you can search for one yourself. Online directories are provided by the American College of Medical Genetics and the National Society of Genetic Counselors. If you need additional help, contact a GARD Information Specialist. You can also learn more about genetic consultations from Genetics Home Reference.