NEET 2013 · Class 12

Principles of Inheritance and Variation

Down's syndrome in humans is due to

  1. (A)three 'X' chromosomes
  2. (B)three copies of chromosome 21
  3. (C)monosomy
  4. (D)two 'Y' chromosomes

The NCERT line that answers it

Verbatim line

Down's syndrome results in the gain of extra copy of chromosome 21.

Chapter
Principles of Inheritance and Variation
Section
4.8.3 Chromosomal Disorders

The answer appears as a continuous sentence in NCERT. This is that sentence.

How this line gives the answer

0, emb=1.0, combined=1.0). Verified: high — The span explicitly states 'Down's syndrome results in the gain of extra copy of chromosome 21', which directly corresponds to option B's 'three copies of chromosome 21' and distinguishes it from the other options.

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