Linkage groups can be separated during . . . . . . in meiosis.

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A)$:
Mendel's law of independent assortment does not hold good for the genes that are located closely on the same chromosome.
Reason $(R)$:
Closely located genes assort independently.
In the light of the above statements,choose the correct answer from the options given below:

$A$ : In block inheritance of all genes located on the same chromosome may occur in some organisms.
$R$ : Dihybrid test cross will have only two phenotypes.

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 $B$.

In a diploid organism,crossing over is responsible for what?

In the fruit fly $Drosophila$,the lack of independent assortment between genes $A$ and $B$ is due to .....