Why did Morgan work with the fruit flies $(Drosophila \text{ } melanogaster)$?

Linked genes are separated by:

Linkage decreases the frequency of

In maize, colored endosperm $(C)$ is dominant over colorless $(c)$ and full endosperm $(R)$ is dominant over shrunken $(r)$. When a test cross is performed on the $F_1$ generation, the following results are obtained:
Colored and Full = $45\%$
Colored and Shrunken = $5\%$
Colorless and Full = $4\%$
Colorless and Shrunken = $46\%$
Based on this data, what is the distance between the two non-allelic genes (in $units$)?

If interference is complete or $100\%$ then the frequency of observed double crossover will be:

Linkage in $Drosophila$ was discovered by: