Linkage was first observed in

In $Neurospora$,the arrangement of $8$ ascospores produced is $2a : 4a : 2a$. This indicates:

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

Which of the following statements are correct?

When a test cross is performed between $F_1$ hybrid flies,the parental type offspring are produced in much higher frequency than the recombinant type offspring. What does this indicate?

The frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes was explained by