When an extra chromosome is added to 21st autosomal chromosome, this leads to the development of which of the following?

Down's Syndrome

The correct answer is Option (3) → Down's Syndrome

• Down's Syndrome occurs due to trisomy of the 21st chromosome (an extra copy of chromosome 21).
• This leads to characteristic features such as intellectual disability, distinct facial features, and sometimes heart defects.

Other options:

• Sickle Cell Disease → caused by a mutation in hemoglobin gene, not a whole chromosome.
• Klinefelter Syndrome → extra X chromosome in males (XXY).
• Haemophilia → caused by mutation on X chromosome, not autosomal.