Question

Down's syndrome in humans is due to

• A. three 'X' chromosomes
• B. three copies of chromosome 21
• C. monosomy
• D. two 'Y' chromosomes.

Answer 1

Answer: (B)

three copies of chromosome 21

Answer 2

Down's syndrome is the trisomy of 21st chromosome in man. Down's syndrome is characterized by short stature, warty skin, protruding tongue, slanting eyes, with folded eyelids. The affected person's face presents a typical mongoloid look. Hence it is also called as mongoloid idiocy. It occurs due to the phenomenon of non-disjunction. Non-disjunction occurs when a pair of homologous chromosomes do not separate in meiosis but migrate to the same pole of the cell resulting in an uneven number of chromosomes in the daughter cells (45 in one and 47 in other) This numerical abnormality results in trisomy (2n + 1) and monosomy (2n - 1). Non-disjunction is more common in sex chromosomes.