Who put forward the "Theory of linkage"?

Thomas Hunt Morgan and his colleagues performed experiments on which organism to verify the chromosomal theory of inheritance and later demonstrated linkage?

$A$ cross is made between two homozygous individuals, one with the wild type $(+, +)$ and the other with the mutant type $(a, b)$. In the $F_2$ generation (or test cross progeny), out of $1000$ individuals, $700$ are parental types. Calculate the distance between genes $a$ and $b$.

Which one of the following pairs is correctly matched?

The lack of independent assortment of two genes $A$ and $B$ in the fruit fly $Drosophila$ is due to:

What is the reason for the absence of hybrids?