Down's syndrome in humans is due to :
- Three 'X' chromosomes
- Monosomy
- Three copies of chromosome 21
- Two 'Y' chromosomes
Answer
(3) Three copies of chromosome 21
Solution
As per NCERT, Down's syndrome is caused by the presence of an additional copy of chromosome number 21 (trisomy of 21).
⇒ The affected individual has 47 chromosomes (2n+1).
∴ Down's syndrome is due to three copies of chromosome 21.
Answer: (3)