If you break down the word trisomy, tri refers to three and somy refers to chromosomes. Since chromsomes occur in pairs, trisomy indicates an additional chromosome at a certain position on a karyotype. Therefore, the answer is chromosome. A trisomy can result during gamete formation. If non-disjunction occurs during meiotic division, then, a gamete may have one more or one less chromosome in it. If the gamete with an extra chromosome goes on to form a zygote, the zygote will contain an additional chromosome at that location. This is a form of aneuploidy which is an abnormal set of chromosomes.