Why did Morgan work with the fruit flies $(Drosophila \text{ } melanogaster)$?

The phenomenon of crossing over in diploid organisms is responsible for

In diploid organisms,crossing over is responsible for which of the following?

Which of the following combinations of characters selected by Mendel shows linkage?

When the number of recombinant progeny is usually less than the number expected in independent assortment, it is called:

Which of the following is true about linkage?
$I.$ It is a phenomenon in which more recombinants are produced in $F_{2}$-generation.
$II.$ More parental combinations are produced in $F_{2}$-generation.
$III.$ Genotypes which are present in $F_{1}$ hybrid reappear in high frequency in $F_{2}$-generation.
$IV.$ It is a phenomenon in which two chromosomes are linked.