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Standard IX
Biology
Allosomes and Autosomes
Question

Down's syndrome is due to
  1. Non-disjunction of chromosomes
  2. Crossing over between genes
  3. Linkage of genes
  4. Sex linked inheritance

A
Crossing over between genes
B
Linkage of genes
C
Sex linked inheritance
D
Non-disjunction of chromosomes
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Solution
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Down's syndrome caused by an error in cell division called "nondisjunction." Nondisjunction results in an embryo with three copies of chromosome number 21 instead of the usual two. Prior to or at conception, a pair of 21st chromosomes in either the sperm or the egg fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. This type of Down's syndrome, which accounts for 95% of cases, is called trisomy 21. Crossover, linkage and sex linked inheritance do not cause down's syndrome. Thus, option A is correct and other options are wrong.

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