Question

In Down's syndrome the chromosome number in each cell of body is

• A. $45$
• B. $47$
• C. $48$
• D. $49$

Answer 1

Answer: (B)

$47$

Down syndrome is caused by having three copies of the genes on chromosome 21, rather than the usual two, resulting in trisomy 21.

Trisomy 21 is caused by a failure of the 21st chromosome to separate during sperm or egg development (nondisjunction). As a result, a sperm or egg cell is produced with an extra copy of chromosome 21. This cell thus has 24 chromosomes. When combined with a normal cell from the other parent, the baby has 47 chromosomes, with three copies of chromosome 21.

So, the correct answer is '47'.