Which mutation causes change in allele?

$A$ : Pseudoalleles are actually closely linked genes.
$R$ : These can be identified easily as both affect different characters, providing separate phenotypes

Frequency of Down’s syndrome increases when the maternal age is

The chromosome shown in the diagram below is broken at the points which are indicated by the arrows and the genes between these points became inverted

The resulting order of the genes will be

Mongolism syndrome is caused by

A medical technician while observing a human blood smear under the microscope notes the presence of barrbody close to the nuclear membrane in the $WBC$. This indicates that person under investigation is