Sex-linked inheritance was discovered by

The linkage map of a fruit fly has a total map distance of $66$ units. The gene for yellow body $(a)$ is at one end and the gene for short hair $(y)$ is at the other end. What will be the recombination frequency between these two genes $(a$ and $y)$?

$A$ : In block inheritance of all genes located on the same chromosome may occur in some organisms.
$R$ : Dihybrid test cross will have only two phenotypes.

Thomas Hunt Morgan worked on which of the following organisms?

The theory of recombination of linked genes due to crossing over of chromosomes during the pachytene stage of meiosis was put forward by:

Drosophila has four pairs of chromosomes. How many linkage groups does it have?