What is Monosomy?

Monosomy is a genetic defect caused by an incomplete set of chromosomes. Various medical conditions are caused by monosomy, some more severe than others. Along with other genetic defects, monosomy can often be identified during prenatal testing, which is why such testing is recommended for women who are at high risk. It is certainly possible for someone to live a healthy and normal life with monosomy, just as other people may be severely disabled by the condition.

Like other chromosomal defects, monosomy is considered a type of aneuploidy. Aneupolidy refers to an abnormality in the chromosomes, leading to birth defects or fetal non-viability. It is most often caused by errors during meiosis, when chromosomes fail to divide properly for any number of reasons. When egg and sperm cells meet, the aneupoloidy results in an incomplete set of chromosomes for the developing fetus. In other cases, the error happens during the complex cell duplication which creates a viable fetus, meaning that the defect is present in some tissues but not in others, a phenomenon called mosaicism.

In complete monosomy, one side of a chromosome pair is simply missing. One example of this type of monosomy is Turner Syndrome, where the X chromosome is missing a complementary chromosome. A number of developmental disabilities are caused by Turner Syndrome, including severe problems like congenital heart disease. However, it is also possible for patients to have a perfectly healthy life with the condition, as long as it is well managed.

Sometimes, part of a pairing chromosome is transferred, causing partial monosomy. In this case, missing genetic information can cause an assortment of problems, including Le Jeune's Syndrome, the result of partial monosomy on chromosome 5. The syndrome is characterized by a number of symptoms, particularly a malformed larynx which causes the voice of the patient to sound strangely high pitched. Developmental disabilities may also be present.

Prenatal testing such as an amniocentesis can reveal monosomy. It is important to receive genetic counseling when undertaking such testing, as test results can be very complicated. After such testing, a doctor will typically call parents in to discuss the results. A negative result indicates that no abnormalities were detected, while a positive result suggests that a problem may be present, and that the parents may have to explore various options for dealing with it. As with all medical testing, false negatives and positives do happen, so parents should always follow up with additional testing and counseling before reaching a decision about how to proceed.