When two genes are situated very close to each other in a chromosome,

Linkage was first studied by .....

Genetic maps of chromosomes are based on the frequency of $:-$

Crossing over in a diploid organism is responsible for

Which organism did $T.H. Morgan$ use in his experiments on linkage and recombination?

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?