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STD 12 - 4. principal of inheritance and variation — BIOLOGY STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceBIOLOGYSTD 12 - 4. principal of inheritance and variation1 Mark
MCQ
Down’s syndrome in humans is due to
  • A
    three $‘X’$ chromosomes
  • ✓
    three copies of chromosome $21$
  • C
    monosomy
  • D
    two $‘Y’$ chromosomes.
✓

Answer

Correct option: B.
three copies of chromosome $21$
b
(b): Down’s syndrome is the trisomy of $21$ stchromosome in man. Down’s syndrome ischaracterized by short stature, warty skin, protrudingtongue, slanting eyes, with folded eyelids. The affectedperson’s face presents a typical mongoloid look. Henceit is also called as mongoloid idiocy. It occurs due tothe phenomenon of nondisjunction.Non disjunction occurs when a pair of homologous chromosomes donot separate in meiosis but migrate to the same poleof the cell resulting in an uneven number ofchromosomes in the daughter cells $(45$ in one and $47$ in other) This numerical abnormality results in trisomy $(2n + 1)$ and monosomy $(2n -1)$. Nondisjunctionismore common in sex chromosomes.

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