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Trisomy 18 (Edwards Syndrome) (cont.)

Trisomy 18 Chromosome Basics

In most cases of trisomy 18, all the cells in the body have an extra copy of chromosome 18. Around 5% of affected people have the extra chromosome 18 in some, but not all, of the body's cells. This phenomenon is known as mosaic trisomy 18. Mosaic trisomy 18 may be very severe or hardly noticeable, depending upon the number of cells that have the extra chromosome.

In rare cases, there is no extra chromosome present; rather, a portion of the long arm of chromosome 18 becomes attached to another chromosome during the formation of egg and sperm cells or very early in development of the embryo. In this case, the individual has two copies of chromosome 18 plus the additional material from chromosome 18 that is attached to another chromosome. This is known as translocation, and the extra genetic material causes the developmental abnormalities in the same way as the presence of an entire extra chromosome. The signs and symptoms of this form of trisomy 18 are dependent upon the amount of chromosomal material that was translocated to another chromosome.

Trisomy 18 is not an inherited condition; it occurs as the result of random events during egg and sperm formation. The type of error that occurs is known as nondisjunction, and this leads to an egg or sperm cell with an abnormal number of chromosomes.

It is not known precisely why the extra genetic material causes the abnormalities specific to trisomy 18. As with Down syndrome (trisomy 21), experts believe that the presence of the extra chromosomal material interferes with the expression and interaction of various genes, resulting in impaired development and function.

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