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

The map distance between genes $A$ and $B$ is $3$ units,between $B$ and $C$ $10$ units,and between $C$ and $A$ $7$ units. The order of the genes in a linkage map constructed on the above data would be:

. . . . . . $(B)$ . . . . . . used the frequency of recombination between gene pairs on the . . . . . . $(A)$ . . . . . . as a measure of the distance between genes and mapped their position on the chromosome.
$(A) \quad\quad (B)$

Two genes '$a$' and '$b$' show $20\%$ recombination. What will be the proportion of gametes produced by a dihybrid $F_1$ individual with the genotype $++/ab$,derived from a cross between $++/++$ and $ab/ab$?

Select the incorrect statement$(s)$ from the options given below with respect to dihybrid cross.
$I.$ Tightly linked genes on the same chromosome show higher recombinations.
$II.$ Genes far apart on the same chromosome show very few recombinations.
$III.$ Genes loosely linked on the same chromosome show similar recombinations.

Which scientist explained linkage and recombination in Drosophila?