The genetic disorder, Down’s Syndrome is caused due to:

The correct answer is Option (3) → Trisomy of chromosome no. 21

Down’s syndrome is a chromosomal disorder. Failure of chromatids to separate in cell division leaves one extra copy of chromosome 21, so the person has 47 chromosomes. This gain of one chromosome is a kind of aneuploidy.

Option (1) Deletion of chromosome no. 21 — Incorrect. Down’s syndrome comes from an extra chromosome, not a lost one.

Option (2) Trisomy of chromosome no. 18 — Incorrect. The extra chromosome is number 21, not 18.

Option (3) Trisomy of chromosome no. 21 — Correct. Down’s syndrome is caused by the presence of an additional copy of chromosome 21, called trisomy of 21.

Option (4) Inversion of chromosome no. 21 — Incorrect. An inversion turns a segment round within the chromosome; it does not add a chromosome.

NCERT Extract:
Chromosomal Disorders – “… Down’s Syndrome : The cause of this genetic disorder is the presence of an additional copy of the chromosome number 21 (trisomy of 21). This disorder was first described by Langdon Down (1866). The affected individual is short statured with small round head, furrowed tongue and partially open mouth (Figure 4.16). Palm is broad with characteristic palm crease. Physical, psychomotor and mental development is retarded.”