is a rare chromosomal abnormality in which there are three copies of the p arm of chromosome 18 in each cell, rather than the usual two copies. As a quick review, chromosomes are structures that hold our DNA. Our DNA contains the instructions, called genes, which tell our bodies how to develop and function. Each chromosome has a constriction, called the "centromere", at one point along its length dividing it into a shorter arm which is called the p (petite) arm and a longer arm, called the q arm. We have two copies of each chromosome, one copy inherited from our mother and the other copy from our father. Therefore, people typically have two copies of chromosome 18, one from mom and one from dad, and thus two copies of the p arm of chromosome 18. (Click here to read more about chromosomes.)
Trisomy 18p has rarely been reported. According to Mabboux et al. (2007) only 21 cases have been documented in the medical literature. The signs and symptoms vary greatly from person to person and is characterized by mental retardation, foot or hand anomalies and craniofacial anomalies. Treatment is based on the specific findings present. Source: Genetic and Rare Diseases Information Center (GARD), supported by ORDR-NCATS and NHGRI.