What is the number of linkage groups in humans with respect to $22$ pairs of autosomes?

The following experiment is the result of a dihybrid cross performed by Morgan. Identify the proportions of $P, Q, R,$ and $S$.

Crossing over is advantageous because it brings about

The presence of recombinants is due to.....

The chromosome maps are not accurate maps because

In $Drosophila$,long wing and broad abdomen characters are dominant over vestigial wing and narrow abdomen characters. $A$ pure-breeding $Drosophila$ with long wings and broad abdomen is crossed with a pure-breeding vestigial-winged and narrow-abdomened $Drosophila$. Two of the $F_1$ individuals were crossed,and the following results were produced in the $F_2$ generation: $482$ $Drosophila$ with long wings and broad abdomen,and $154$ $Drosophila$ with vestigial wings and narrow abdomen. Which one of the following is illustrated by these results?