<unk>  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<unk>  may occur with a higher frequency in children of older mothers, there is not enough evidence to say that this is definitively the case.
Tetrasomy 18p
Tetrasomy 9p
Monosomy 9p
Distal trisomy 10q
2q37 monosomy
Partial monosomy 13q
Small supernumerary marker chromosome
Chromosome 2q deletion