In maize,the chromosome number is $2n = 20$. The number of linkage groups in it shall be:

Genes $A, B$,and $C$ are linked. Genes $A$ and $B$ are closer than $A$ and $C$. Find out the correct option for the given statements:
$I.$ $A$ might be before $B$ and $C$
$II.$ $B$ might be between $A$ and $C$
$III.$ $C$ might be between $A$ and $B$
$IV.$ More crossovers have occurred between $A$ and $C$ than $A$ and $B$.

Frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes to map their position on the chromosome was used for the first time by

Frequency of crossing over in a chromosome is an index of

Crossing over causes

Give differences: Crossing over and Linkage.