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

  $75 \%$
• Correct answer
  C

  $50 \%$
• D

  $25 \%$

Solution

Down 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 $(45+X X)$ and normal father has $(44+X Y)$
Thus, progeny $=50 \%$ trisomics and $50 \%$ normal child

Hence, the correct answer is option: 'C'.