A) trisomy of 13th chromosome
B) non disjunction of 21st chromosome
C) trisomy of 18th chromosome
D) fusion of normal sperm with egg having 2X chromosome
Correct Answer: B
Solution :
Downs syndrome (mongolism) is due to trisomy of 21st chromosome. It is caused by non-disjunction of 21st chromosome pair during anaphase-I as result (n+1) gamete is formed. The fusion of this gamete with normal gamete result in trisomy. Patauis syndrome is due to trisomy of 13th chromosome. Edwards syndrone is due to trisomy of 18th chromosome. When a normal sperm fuse with egg having two X chromosomes, the offspring with XXY sex complement is formed. This is called Klinefelters syndrome.
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