Monosomy and trisomy can be represented as

Which disorder is indicated by the following karyotype?

$A$ man having Klinefelter's syndrome is

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

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?

The Barr body is observed in