Question

Given below are two statements:
Statement I: Down's syndrome is caused by the absence of one of the X-chromosomes.
Statement II: Turner's syndrome is caused by the presence of an additional copy of the chromosomes.
In the light of the above statements, choose the correct answer from the options given below:

• A. Statement I is incorrect but Statement II is correct
• B. Both Statement I and Statement II are correct
• C. Both Statement I and Statement II are incorrect
• D. Statement I is correct but Statement II is incorrect

Hint:

Down's syndrome results in a total of \(47\) chromosomes due to autosomal trisomy. Turner's syndrome results in a total of \(45\) chromosomes due to sex chromosome monosomy. Klinefelter's syndrome is another key sex-chromosome aneuploidy, involving an extra X chromosome (\(XXY\), \(47\) chromosomes).

Answer 1

The Correct Option is C

Step 1: Read the statements.
Statement I: Down's syndrome is caused by absence of one X-chromosome. Statement II: Turner's syndrome is caused by an extra copy of chromosomes.
Step 2: Recall aneuploidy.
Aneuploid disorders arise from gain or loss of chromosomes due to non-disjunction during cell division.
Step 3: Recall Down's syndrome.
Down's syndrome is caused by an extra copy of autosome 21 (Trisomy 21), not by losing an X chromosome.
Step 4: Judge Statement I.
Since Down's is a gain of chromosome 21 and not loss of an X, Statement I is incorrect.
Step 5: Recall Turner's syndrome.
Turner's syndrome is caused by the absence of one X chromosome, giving an $XO$ (45 chromosomes) karyotype, which is a loss, not a gain.
Step 6: Judge Statement II and conclude.
Since Turner's is a loss and not an extra copy, Statement II is also incorrect. Both statements are incorrect, which is option (3).
\[ \boxed{\text{Both Statement I and Statement II are incorrect}} \]