Down’s syndrome in humans is due to - Vidyadip

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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