Question

Extra copy of 21st chromosome causes _________ in human.

• A. Turner's syndrome
• B. Klinefelter's syndrome
• C. Thalassemia
• D. Down's syndrome

Hint:

Trisomy of chromosome 21 gives a well-known syndrome.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept
An extra chromosome in a pair is called trisomy and arises from non-disjunction.

Step 2: Trisomy 21
Three copies of chromosome 21 give 47 chromosomes and cause Down's syndrome.

Step 3: Compare the options
Turner's syndrome is 45, XO, Klinefelter's syndrome is 47, XXY, and thalassemia is a single-gene blood disorder.

Final Answer:
An extra chromosome 21 causes Down's syndrome. \[ \boxed{\text{(D) }\text{Down's syndrome}} \]