A genetic abnormality in which a diploid organism has an extra copy of one of its chromosomes. Normally, diploid organisms (such as humans and most other vertebrates, as well as many flowering plants) have two copies of each chromosome in their genome. An individual with trisomy has three copies of one of its chromosomes.

Trisomy occurs when one of the haploid gametes that fuse to form a zygote has an extra chromosome. A gamete can end up with an extra due to nondisjunction at the first division of meiosis, when a tetrad fails to separate and both homologous chromosomes go into the same daughter cell. Also, nondisjunction at the second meiotic division (when a pair of chromatids fail to separate, and both go to the same side) can also result in a gamete having an extra chromosome.

Quite often, trisomy means the individual can't survive. Some types of trisomy, however, aren't so deleterious, and the individual can survive. In humans, examples are Trisomy 21 (also known as Down's Syndrome and Trisomy 13.