Who clearly proved and defined linkage?

Linkage in $Drosophila$ was discovered by:

. . . . . . $(B)$ . . . . . . used the frequency of recombination between gene pairs on the . . . . . . $(A)$ . . . . . . as a measure of the distance between genes and mapped their position on the chromosome.
$(A) \quad\quad (B)$

$T.H.$ Morgan worked with $Drosophila$ to prove the chromosomal theory of inheritance. Which of the following characteristics was $NOT$ present in $Drosophila$?

Which event during cell division is responsible for the emergence of new characteristics in the next generation of organisms?

Primary source of allelic variation is