$A$ disease caused by an autosomal primary non-disjunction is

Euploidy is best explained by

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 $\%$)?

If there is an addition of a single chromosome to the total set of chromosomes,it is called . . . . . . .

The number of chromosomes in Down's syndrome is

In which disorder is the $XXY$ karyotype present?