How are changes in chromosome 18 related to health conditions?
Trisomy 18 occurs when each cell in the body has three copies of chromosome 18 instead of the usual two copies. Trisomy 18 can also result from an extra copy of chromosome 18 in only some of the body’s cells (mosaic trisomy 18). Rarely, trisomy 18 occurs when the long (q) arm of chromosome 18 becomes attached (translocated) to another chromosome during the formation of reproductive cells (eggs and sperm) or very early in embryonic development. Affected individuals have two copies of chromosome 18, plus extra material from chromosome 18 attached to another chromosome. People with this genetic change are said to have translocation trisomy 18. If only part of the q arm is present in three copies, the physical signs of translocation trisomy 18 may be different from those typically seen in trisomy 18. If the entire q arm is present in three copies, individuals may be as severely affected as if they had three full copies of chromosome 18. Researchers believe that extra copies of some genes o