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 $b)$?

Generation of non-parental gene combinations is termed as

Define/Explain the following terms:
$1.$ Linked genes
$2.$ Locus

The distance between two linked genes on a chromosome is measured in cross-over units,which is:

When normal and mutant alleles are present on opposite chromosomes of a homologous pair,the heterozygotes are called as:

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 $B$.