Who put forward the "Theory of linkage"?

Morgan and his group observed that when genes are grouped on the same chromosome,some genes are very tightly linked and show:

The first mutant gene reported by $T.H. Morgan$ in $Drosophila$ was the . . . . . . .

Which of the following does not follow the law of independent assortment?

Which is the most common mechanism of genetic variation in the population of a sexually reproducing organism?

$A$ pure breeding pea plant with the dominant phenotype of character $A$ and recessive phenotype of character $B$ was crossed with another pure breeding plant with the recessive phenotype of $A$ and dominant phenotype of character $B$. An offspring of this cross was crossed with a homozygous recessive for both the characters $A$ and $B$. The following results were obtained:
$22$ were phenotypically dominant for $A$ and recessive for $B$.
$4$ were phenotypically dominant for both $A$ and $B$.
$4$ were phenotypically recessive for both $A$ and $B$.
$24$ were phenotypically recessive for $A$ and dominant for $B$.
These results show that: