Linked genes are present on

Mendel observed that some traits do not assort independently. Later research showed that this is due to .....

Linkage was first studied by .....

When is the Law of Independent Assortment not applicable to genes?

What is the reason for the absence of hybrids?

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 $units$)?