No. It is change in chromosome number. If a gamete has 2 alleles for a trait, it must have 2 chromosomes for that trait. When that gamete joins with another gamete, the result will be three chromosomes, which is one too many. Remember, each normal gamete carries one chromosome per pair, and thus has one allele per trait. That way, when fertilization happens, there are two chromosomes per pair and thus two alleles per trait.