Delayed selection strategy to identify chromosome IV disomes. (A) Cells were grown approximately one doubling in the presence of galactose to induce nondisjunction, diluted and plated to YPD rich medium to allow all viable cells to form colonies. If any colonies are clones of stable disomes, they should produce Ura+His+ papillae at high frequency when replica-plated to selective medium. Spontaneous duplications that occur after colony formation on YPD should appear as isolated papillae on selective medium. (B) Photographs of replica plates after 3 days. Medium lacks uracil and histidine. Triangles indicate isolated papillae appearing on non-growing "ghost colonies." Arrows indicate single colonies on which numerous papillae appear. Strain shown is KAY614. Bar is 5 mm.