Jon and Lily are parents of child A; Bob and Jill are parents of child B; and Don and Jess are parents of child C. Children A, B, and C have Down syndrome. A Southern blot is used to analyze the restriction fragment length polymorphisms (RFLPs) of the parents and their children. Since trisomy 21 is the major cause of Down syndrome, a probe complementary to a sequence of chromosome 21 is used for RFLP detection. Thicker bands on the blot indicate the presence of two RFLPs of the same fragment length.

Label each child with the non-disjunction event that produced their RFLP pattern.

FELE Jon Lily child A Bob Jill child B Don Jess child C meiosis II in father meiosis I in mother undetermined meiosis I in father meiosis II in mother