Down's syndrome is caused by the trisomy of which chromosome pair?

Down syndrome is caused by an extra copy of chromosome $21$. If an affected mother and a normal father produce offspring,what percentage of the offspring will be affected by this disorder (in $\%$)?

The $Cri-du-chat$ syndrome is caused by a chromosomal structural change involving:

$A$ condition characterized by not having an exact number of chromosomes in a diploid set is called

Which of the following characteristics is common in plants?

The Barr body is observed in