The cause of Down’s syndrome is:

The correct answer is Option (2) → Trisomy of chromosome 21

Down’s syndrome is a chromosomal disorder. It is caused by an additional copy of chromosome number 21 (trisomy of 21). The cells therefore carry 47 chromosomes instead of 46. It was first described by Langdon Down in 1866. The affected person is short, with a small round head, a furrowed tongue and a partially open mouth. The palm is broad with a characteristic crease, and physical, psychomotor and mental development is slow.

Chromosomes 18, 11 and 16 are not the chromosomes involved in this syndrome.

Note: A similar question was also asked in CUET (UG) 2023 on 01-06-2023 (Slot 3).

NCERT Extract:
Chromosomal Disorders –

Figure 4.16 A representative figure showing an individual inflicted with Down’s syndrome and the corresponding chromosomes of the individual