trisomy 9 trisomy 9 full trisomy 9 is a rare and fatal chromosomal disorder caused by having three copies (trisomy) of chromosome number 9. it can be a viable condition if trisomy affects only part of the cells of the body (mosaicism) or in cases of partial trisomy (trisomy 9p) in which cells have a normal set of two entire chromosomes 9 plus part of a third copy, usually of the short arm of the chromosome (arm p). trisomy 9chromosome 9specialtymedical genetics causesthree copies of chromosome 9prognosisinvariably fatal none treatment