Philadelphia chromosome is

What is the cause of $Down's$ syndrome?

The condition of sex chromosomes in a male child with Down's syndrome will be:

The number of chromosomes in Klinefelter's syndrome is

The human chromosomes are divided into $7$ groups. $B$-chromosomes are:

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?