Define/Explain: Linkage.

Which of the following characteristics cannot be observed in the progeny if linkage is present?

Mendel did not recognize the linkage phenomenon in his experiments because

In a fruit fly,the $X$-chromosome linkage map has a total length of $66$ units,with the yellow body gene $(y)$ at one end and the bobbed hair gene $(b)$ at the other end. What will be the recombination frequency between these two genes $(y$ and $b)$?

The linkage map of $X$-chromosome of fruit fly has $66$ units with yellow body gene $(y)$ at one end and bobbed hair $(b)$ gene at the other end. The recombination frequency between these two genes ($y$ and $b$) should be

Who clearly proved and defined linkage?