In which of the following can recombination be observed?

Sex linkage was first observed by.....

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.

$A$ genetic map is one that

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

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