Medical Genetics (2) Aneuploidy is the condition of having less than or more than the normal diploid number of chromosomes, and is the most frequently observed type of cytogenetic abnormality. In other words, it is any deviation from euploidy although many authors restrict use of this term to conditions in which only a small number of chromosomes are missing or addedMedical Genetics (2) Aneuploidy is the condition of having less than or more than the normal diploid number of chromosomes, and is the most frequently observed type of cytogenetic abnormality. In other words, it is any deviation from euploidy, although many authors restrict use of this term to conditions in which only a small number of chromosomes are missing or added