Which of the following is an example of a non-Mendelian disorder?

In humans,sometimes during gametogenesis,sex chromosomes do not separate,resulting in a chromosome number of $45$,$47$,or $48$. In this condition,which of the following genotype and phenotype combinations is correct?

If the haploid chromosome number in a cell is $12$,then the monosomic number will be:

The number of sex chromosomes is normal in

Which pair of chromosomes is concerned with Down's syndrome?

Down's syndrome occurs in the $21^{st}$ pair of chromosomes due to the presence of an additional chromosome.