A karyotypically normal couple has a child with trisomy 21 D

A karyotypically normal couple has a child with trisomy 21 Down syndrome. DNA analysis if 21q polymorphic markers reveals the following results. (Letters refer to polymorphic loci; numbers refer to alleles at that locus.) In which meiotic division did nondisjunction occur? Marker Locus Father Mother child A 1, 2 2, 2 1, 2, 2 B 1, 1 2, 2 1, 2, 2 C 1, 1 1, 1 1, 1, 1 D 1, 1 1, 2 1, 2, 2 (A) Maternal meiosis I (B) Maternal meiosis II (C) Paternal meiosis I (D) Paternal meiosis II (E) Paternal meiosis I or II

Solution

maternal meosis 1 because it leads to trisomy of the chromosome 21.

Nondisjunction in maternal meosis 1 leads to abnormal chromosome21