Principles of Inheritance and Variation Question 190
190. In Down’s syndrome, Karyotyping has shown that the disorder is associated with trisomy of chromosome number-21 usually due to [1998]
(a) non-disjunction during egg-cell formation
(b) non-disjunction during sperm-cell formation
(c) non-disjunction during formation of eggcells and sperm-cells
(d) addition of extra chromosome during mitosis of the zygote
Show Answer
Answer : c
Hints & Solution
(c) Down’s syndrome caused by trisomy 21, arises due to an occassional non disjunction during meiosis when a gamete comes to possess an extra chromosome $(n+1)$. Fusion with a normal gamete $[n+(n+1)]$ produces trisomy.
