Solution:
Nondisjunction is the result of the paired homologous chromosomes seperating unequally during meiosis. So instead of one of a particular chromosome going to each of the 4 resulting sex cells, one cell gets two chromosomes of that type. Then during fertilization that zygote gets three of that particular chromosome instead of the normal two, resulting in one extra chromosome. That zygote then undergoes mitosis, resulting in all of that organisms cells containing one extra chromosome.
