Question

Study the following and pick up the correct statements.
I. Klinefelter's syndrome is an example for trisomy in allosomes.
II. Turner's syndrome is due to monosomy in allosomes.
III. Down's syndrome is due to trisomy of $21^{\text{st}}$ chromosome.
IV. Aneuploidy is due to non-disjunction during gametogenesis.

• A. I only
• B. I and II only
• C. I, II and III only
• D. I, II, III and IV

Hint:

Remember the chromosomal numbers of key genetic disorders:
Down's syndrome = Autosomal trisomy (chromosome 21).
Klinefelter's syndrome = Allosomal trisomy (XXY).
Turner's syndrome = Allosomal monosomy (XO).
All of these arise due to meiotic non-disjunction of chromosomes.

Answer 1

The Correct Option is D

Step 1: Verify statement I and II.
Klinefelter's syndrome gives an extra X chromosome, an allosomal trisomy, so I is correct. Turner's syndrome gives only one X chromosome, an allosomal monosomy, so II is correct.
Step 2: Verify statement III.
Down's syndrome is the well known case of an extra copy of chromosome 21, an autosomal trisomy, so III is correct too.
Step 3: Verify statement IV.
Aneuploidy in general arises when chromosomes fail to separate properly during gamete formation, non-disjunction, which covers all the above disorders, so IV is correct as well.
Final answer: Option 4, all four statements I, II, III and IV are correct.