Isodicentric 15 - Genetics

Genetics

Generally, idic(15) is not inherited; it is said to appear de novo; in one member of the family, by chance. In most cases, the abnormal chromosome is generated in the mother's germ cells; oocytes. This finding is due to ascertainment bias; i.e. cases with maternally derived idic(15) usually have clinical findings and attract attention but those with paternally derived idic(15) usually do not. Thus, established cases are usually patients with maternally inherited idic (15).

People with idic(15) have extra genetic material that has developed from chromosome 15. The material usually exists as a little extra chromosome 15; which is called marker chromosome or extra structurally abnormal chromosome (ESAC). The marker usually exists as an isodicentric chromosome; i.e. 2 copies of a specific part of the long arm of chromosome 15 (most commonly, 15q11-q13) that is mirrored and doubled, with 2 centromeres and 2 DNA satellites. The smallest markers appear to be harmless and they may go undetected. However, if they are large enough to contain a number of important genes, they may result in "idic(15) syndrome" which is characterized by mental retardation, autism and other neurological symptoms. One of the regions responsible for the symptoms of idic(15) syndrome is the critical PWS/AS-region named after the Prader-Willi and/or Angelman syndromes.