Frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes to map their position on the chromosome was used for the first time by

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

Frequency of crossing over in a chromosome is an index of

What is the reason for the absence of hybrids?

In a certain plant, red flowers $(R)$ are dominant to white flowers $(r)$ and tallness $(T)$ is dominant to dwarfness $(t)$. $A$ heterozygous plant $(RrTt)$ is backcrossed with a double recessive plant $(rrtt)$. If the gene loci for colour and size are situated very close together on the same chromosome, the expected percentage of offspring could be approximately:

Tall Red

Tall White

Dwarf Red

Dwarf White

respectively.

Define/Explain the following terms:
$1.$ Linked genes
$2.$ Locus