Down's syndrome is caused by an extra copy of chromosome number 21. What percentage of offspring produced by…
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?
$100 \%$
$75 \%$
$50 \%$
$25 \%$
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
Thus, progeny $=50 \%$ trisomics and $50 \%$ normal child