Mendel observed that some characters did not assort independently. Later researchers found it to be due to

$A$ test cross of an $F_1$ fly $+a/+b$ produces the following offspring:
$++/ab = 9$
$ab/ab = 9$
$+b/ab = 41$
$a+/ab = 41$
What is the distance between the linked genes?

Linkage was first studied by .....

Describe linkage and recombination in detail.

The distance between two genes in a chromosome is measured in cross-over units,which represent:

$A$: Morgan's cross $III$ was conducted in $Drosophila$ to locate genes on the chromosome for white eye color.
$R$: The cross was done between red-eyed hybrid female and white-eyed male.