When the chromosome number of a given organism has one additional chromosome in one of the homologous pairs,the addition is known as

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

Which of the following disorders is related to the karyotype given below?

The exchange of chromosomal segments between two non-homologous pairs of chromosomes is known as:

$A$ human female with Turner'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?