Gynecomastia is a symptom of

Mark the correct match.

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?

What is the number of Barr bodies in an individual with Klinefelter syndrome?

Down's syndrome is an example of

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