Question

Clarify the term non-disjunction giving a suitable example to substantiate the same. [2 MARKS]

Answer 1

Clarification: 1 Mark
Example: 1 Mark

When a homologous pair of chromosome fails to split during gamete formation, one gamete has an extra chromosome while the other gamete is deficient in one chromosome. This is known as nondisjunction.
For example, Mongolism or Down's syndrome results due to trisomy of 21st chromosome. In some cases, the 21st pair of chromosome does not split during oogenesis in females and one egg is produced with two 21st chromosomes. When this ovum is fertilized by a normal sperm, the zygote will have three 21st chromosomes resulting in genetic abnormally known as Mongolism or Down's Syndrome.