How do twinned spots arise? Could it be caused by two ND events happening in adjacent cells?
Twinned spots arise through mitotic recombination in a cell heterozygous for 2 traits. Nondisjunction will result in one mutant patch, while mitotic recombination will invariably give twinned spots (2 mutant patches of different phenotypes right next to one another and of similar size). Two ND events occurring in neighboring cells to give twinned spots would be far less likely, as nondisjunction is quite rare.
