testing. Genetics Tetrasomy 18p is caused by the presence of an additional isochromosome composed of two copies of the p arm of chromosome 18. This extra chromosome is classified as a small supernumerary marker chromosome that forms de novo in a parent's egg or sperm or, in rare cases, is directly inherited from a parent carrier of the intact small supernumerary marker chromosome. It has been reported in both the non-mosaic as well as the mosaic state. (The phrase "mosaicism" in this context means that some cells carry the genetic change while others do not.) In the grand majority of cases, the isochromosome is de novo. Although there has been some speculation that tetrasomy 18p may occur with a higher frequency in children of older mothers, there is not enough evidence to say that this is definitively the case. Diagnosis Suspicion of a chromosome abnormality is typically raised due to the presence of developmental delays or congenital malformations. Diagnosis of tetrasomy 18p is typically made via a routine chromosome analysis from a blood sample. The diagnosis can also be made prenatally by chorionic villus sampling or amniocentesis. Severity of tetrasomy 18p is variable. Individuals with mosaicism are typically less severely affected than non-mosaic individuals. Management At present, treatment for tetrasomy 18p is symptomatic, meaning that the focus is on treating the signs and symptoms of the conditions as they arise. The Chromosome 18 Clinical Research Center has published a list of recommended screening and evaluations: Research Current research is focusing on clearly defining the phenotype associated with tetrasomy 18p and identifying which genes cause medical and developmental problems when present in four copies. References External links Genetic disorders with no OMIM