Question

Down's syndrome is caused by an extra copy of chromosome number 21. What percentage of offspring produced by an affected mother and a normal father would be affected by this disorder?

• A. 50%
• B. 25%
• C. 100%
• D. 75%

Answer 1

The correct option is A 50%

Down's syndrome is an autosomal disorder and is caused by an extra copy of chromosome number 21. According to the question, affected mother carries 47 chromosomes (45+XX) and normal father has (44+XY).

Gametes 45 +XX---> 44+XY	23 +X	22 +X
22 +X	45+XX (diseased)	44+XX (normal)
22 +Y	45+XY (diseased)	44+XY (normal)

Thus, progeny = 50% trisomics : 50% normal child;

So, the correct answer is '50%'