<unk>  is caused by the presence of two additional copies of the short arm of chromosome 9. These two extra copies are found in the cell as an isochromosome, in addition to the normal 46 chromosomes. An isochromosome is formed when one of the arms of a chromosome is duplicated (in this case, the short arm), and the other is lost (in this case, the long arm), forming a chromosome with two identical arms. Varying amounts of the short arm may be incorporated into the isochromosome, and occasionally, small regions of DNA from the long arm are included as well. This extra isochromosome is classified as a small supernumerary marker chromosome. The disorder is almost never inherited; it most commonly arises through the improper distribution of chromosomes during the formation of eggs or sperm.
Tetrasomy 9p
Tetrasomy 18p
Monosomy 9p
Distal trisomy 10q
Partial monosomy 13q
2q37 monosomy
Small supernumerary marker chromosome
Xp11.2 duplication