Question

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

• A. 100%
• B. 75%
• C. 50%
• D. 25%

Answer 1

Answer: (C)

50%

Down's syndrome is an autosomal disorder and is caused by an extra copy of chromosome number 21. According to the question, the 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 option C.