Question

In humans, non-disjunction of the 21st pair of chromosomes leads to:

• A. Acquired Immune Deficiency Syndrome
• B. Klinefelter’s Syndrome
• C. Turner’s Syndrome
• D. Down’s Syndrome

Hint:

Trisomy 21 → Down’s Syndrome, Trisomy 18 → Edward’s Syndrome, XO → Turner’s, XXY → Klinefelter’s.

Answer 1

The Correct Option is D

Step 1: Non-disjunction refers to the failure of homologous chromosomes to separate during meiosis.
Step 2: When this occurs in the 21st chromosome pair, it results in an extra copy (trisomy 21).
Step 3: This leads to a total of 47 chromosomes in the zygote instead of the normal 46.
Step 4: The child exhibits symptoms of Down’s Syndrome: mental retardation, flattened facial profile, and short stature.
Step 5: Other syndromes result from different chromosomal abnormalities (e.g., XXY for Klinefelter’s, XO for Turner’s).
\[ \Rightarrow \text{Correct answer is } \boxed{\text{(D) Down’s Syndrome}}. \]