Nondisjunction in meiosis causing trisomy 21

Steps:

• Meiosis normally halves chromosomes to 23 per gamete for proper fertilization.
• Down's syndrome results from trisomy 21, an extra chromosome 21 in the child.
• If a gamete has 24 chromosomes (extra 21), fertilization with a normal 23-chromosome gamete yields 47 chromosomes total.
• This extra chromosome 21 directly causes Down's syndrome traits.

Why A is correct:

• Trisomy 21, defining Down's syndrome, occurs when a gamete with an extra chromosome 21 fuses with a normal one, per Mendelian inheritance principles.

Why the others are wrong:

• B: Lacking one gene may cause monogenic disorders but not the chromosomal trisomy of Down's.
• C: Lacking one chromosome leads to monosomy (e.g., Turner syndrome), not trisomy.
• D: An extra gene alters a single locus but does not produce the full extra chromosome seen in Down's.

Final answer: A