Principles of Inheritance and Variation Question 152
152. Down’s syndrome in humans is due to
[NEET Kar. 2013]
(a) Two ’ $Y$ ’ chromosomes
(b) Three ’ $X$ ’ chromosomes
(c) Three copies of chromosome 21
(d) Monosomy
Show Answer
Answer : c
Hints & Solution
(c) Down’s syndrome is the chromosomal disorder due to the presence of an additional copy of the chromosome number 21 (trisomy of 21). The affected individual is short statured with small round head, furrowed tongue and partially open mouth and mental development is retarted. Down syndrome is caused due to nondisjunction can occur during anaphase of mitosis, meiosis I, or meiosis II. During anaphase, sister chromatids (or homologous chromosomes for meiosis I), will separate and move to opposite poles of the cell, pulled by microtubules. In nondisjunction, the separation fails to occur.
