Down syndrome is caused by

Answer and explanation

Correct answer: C. trisomy of chromosome 21

  1. Step 1: Down syndrome is a congenital chromosomal aneuploidy resulting from nondisjunction during parental meiosis.
  2. Step 2: The failure of chromosome 21 homologs to disjoin leads to an extra copy, creating trisomy 21 with a total chromosome count of ().
  3. Step 3: Monosomy X causes Turner syndrome, whereas chromosome 5 deletion causes cri-du-chat syndrome.
Answer
trisomy of chromosome 21.
Why not the others: monosomy of the X chromosome causes Turner syndrome (); a deletion in chromosome 5 causes cri-du-chat syndrome (); trisomy of chromosome 13 causes Patau syndrome.

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