Briefly mention the contribution of $T.H.$ Morgan in genetics.

(N/A) $T.H.$ Morgan's work was primarily based on the fruit fly, $Drosophila$ $melanogaster$.
He formulated the chromosomal theory of linkage.
He defined linkage as the physical co-existence of two or more genes on the same chromosome.
He performed dihybrid crosses in $Drosophila$ to demonstrate that linked genes are inherited together and are located on the $X$-chromosome.
His experiments also proved that tightly linked genes show very low recombination frequencies, whereas loosely linked genes show higher recombination frequencies.