Which one of the following is a chromosomal disorder?

Three copies of chromosome-$21$ in a child with Down's syndrome have been analysed using molecular biology technology to detect any possible $DNA$ polymorphism with reference to different alleles located on chromosome-$21$. Results showed that out of $3$ copies,$2$ of the chromosomes of the child contain the same alleles as one of the mother's alleles. Based on this,when did the non-disjunction event most likely occur?

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

The Philadelphia chromosome is found in patients suffering from

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

Who reported that Down's syndrome is due to an extra $21^{st}$ chromosome?