The tetrasomy is typically caused by the incorrect distribution of chromosomes during meiosis or mitosis, called nondisjunction. When cell division occurs normally, each daughter cell receives one short arm and one long arm of each chromosome. However, errors during this process may cause one daughter cell to receive two short arms of chromosome 9, while the other cell receives two long arms. The identical arms are subsequently connected via a centromere. In most cases, isochromosomes of 9p contain two centromeres, called a dicentric chromosome. The tetrasomy can also be formed independently of cell division. Double stranded breaks in the short arm of chromosome 9 may be repaired incorrectly, resulting in the formation of an isochromosome of 9p with a single centromere. This isochromosome can then be passed on during cell division. Mosaicism In most cases, affected individuals carry the tetrasomy in every cell in their bodies. However, some patients have the tetrasomy in some of their tissues but not in others; this is referred to as the mosaic form of the syndrome, and often results in less severe symptoms. Non-mosaic<unk>  is most often the result of abnormal chromosome separation during the formation of eggs or sperm. In contrast, the mosaic form is often a result of a nondisjunction event that occurs early in embryonic development. The type and number of tissues affected in the mosaic form is dependent upon the timing and location of the abnormal division within the developing embryo.
Tetrasomy 9p
Tetrasomy 18p
Monosomy 9p
Distal trisomy 10q
Partial monosomy 13q
2q37 monosomy
Small supernumerary marker chromosome
Xp11.2 duplication