Multiple choice

Molecular analysis is performed on the three copies of chromosome 21 in a child with Down syndrome using DNA polymorphisms for which both parents are heterozygous for different alleles. Two of the chromosomes contain same alleles as one of the mother's alleles. Based on this, when did the nondisjunction event most likely occur?

  1. Maternal meiosis I

  2. Maternal meiosis II

  3. Paternal meiosis I

  4. Paternal meiosis II
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

If two copies of chromosome 21 have the same maternal alleles, the nondisjunction must have occurred in maternal meiosis II, when the two identical chromatids normally separate.

Keep practicing — related questions