Figure 1 .

Model for nondisjunctional segregation with proper congression. A heterologous segregation event is drawn here; black arrows indicate chromosome movements. (A) If chromosomes are out on the spindle in a maloriented configuration, and complete congression without correcting the defect, then (B) the metaphase-arrested chromosome mass would contain all the chromosomes, yet will have both homologs pointed toward the same spindle pole. (C) When sister chromatid cohesion along chromosome arms is dissolved at anaphase, the maloriented chromosomes would proceed to opposite poles. If the left pole becomes the pronucleus, the oocyte will be diplo-X nullo-4, while the other pole would produce nullo-X diplo-4.