Question

Down's syndrome which is the trisomy of 21${}^s$${}^t$ chromosome pair is due to

• A. Non-disjunction
• B. Linkage
• C. Crossing over
• D. Mutations

Answer 1

Answer: (A)

Non-disjunction

The linkage is the tendency of closely placed genes on a chromosome to assort together and not to show independent assortment. It forms new gene combinations but can not duplicate a chromosome as is the case in Down's syndrome. Cross over is the exchange of genetic material between non-sister chromatids of homologous chromosomes. Again, it can create new gene combination but can not duplicate a chromosome as is the case in Down's syndrome. Mutation is random heritable changes in the genetic material that can duplicate a chromosome and can cause Down's syndrome. Non-disjunction of a bivalent during meiosis produce gametes with "n+1 and n-1". Fertilization of "n+1 " gamete with a normal gamete produce "2n+1" individual. Hence, non-disjunction is the direct cause of Down's syndrome. Thus, the correct answer is option A.