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. 100%
• B. 75%
• C. 50%
• D. 25%

Answer 1

Answer: (C)

50%

Answer 2

Down's syndrome is typically caused by the presence of an extra copy of chromosome 21. It is most commonly associated with nondisjunction during meiosis, leading to an extra chromosome 21 in the resulting offspring.
When an affected mother (who has two copies of chromosome 21) and a normal father (who has two copies of chromosome 21) have offspring, the possible combinations of chromosome inheritance are as follows:

1. Normal offspring (2 copies of chromosome 21 from each parent): 25%
2. Offspring with Down's syndrome (3 copies of chromosome 21 from the mother, 1 from the father): 50%
3. Offspring with Down's syndrome (3 copies of chromosome 21 from the father, 1 from the mother): 25%

So, 50% of the offspring produced by an affected mother and a normal father would be affected by Down's syndrome.

Therefore, the correct answer is (C): 50%.