Nondisjunction. A rare occurrence during meiosis, nondisjunction produces gametes that have either one extra chromosome or one fewer chromosome. (a) Nondisjunction of the X chromosome is shown here; the white-eye allele is on the X chromosome. (b) Fertilization with normal sperm produces adult flies with odd numbers of chromosomes, as illustrated in the Punnett square. (c) Bridges predicted the occurrence of nondisjunction to explain rare progeny phenotypes, and cytologic examination of chromosomes in the rare progeny confirmed the predicted extra Y chromosome in a white-eyed female.