$A$ person who is trisomic for the $21^{st}$ pair of chromosomes has:

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?

$A$ chromosome complement with $2n-1$ is called as:

Mongolism syndrome is caused by

Sometimes chromosome number increases or decreases due to:

Which disorder is indicated by the following karyotype?