Gynecomastia is a symptom of

The number of $Barr$ bodies in $XXXXY$ is .....

Down's syndrome occurs in the $21^{st}$ pair of chromosomes due to the presence of an additional chromosome.

Which one of the following disorders $CANNOT$ be treated by gene therapy?

$A$ male individual with $47$ chromosomes due to the addition of an $X$ chromosome suffers from a condition called:

Down syndrome is caused by an extra chromosome in the $21$st pair. What percentage of offspring from an affected mother and a normal father will show the effect of this disorder (in $\%$)?