When an extra chromosome is added to 21st autosomal chromosome, this leads to the development of which of the following?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Down’s Syndrome
Down’s syndrome is caused by an extra copy of chromosome 21 (trisomy 21), so the person has 47 chromosomes instead of 46. It usually arises when the chromosome 21 pair fails to separate during the formation of the egg or sperm. It causes short stature, a broad flat face, a furrowed tongue and delayed mental development.
Why the other options are incorrect
Option (1) Sickle Cell Disease — Incorrect. A gene mutation in the haemoglobin gene that makes red blood cells sickle-shaped; the number of chromosomes is normal.
Option (2) Klinefelter Syndrome — Incorrect. An extra X chromosome in a male (44 + XXY), a change in the sex chromosomes, not in chromosome 21.
Option (4) Haemophilia — Incorrect. An X-linked gene disorder in which the blood does not clot properly; the number of chromosomes is normal.