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

Discuss in detail the contributions of Morgan and Sturtevant in the area of genetics.

The recombination frequency between genes $B$ and $A$ is $5\%$. The recombination frequency between genes $A$ and $C$ is $15\%$. What is the possible sequence of these genes on the chromosome?

$A$ pure breeding pea plant with the dominant phenotype of character $A$ and recessive phenotype of character $B$ was crossed with another pure breeding plant with the recessive phenotype of $A$ and dominant phenotype of character $B$. An offspring of this cross was crossed with a homozygous recessive for both the characters $A$ and $B$. The following results were obtained:
$22$ were phenotypically dominant for $A$ and recessive for $B$.
$4$ were phenotypically dominant for both $A$ and $B$.
$4$ were phenotypically recessive for both $A$ and $B$.
$24$ were phenotypically recessive for $A$ and dominant for $B$.
These results show that:

$A$ test cross of an $F_1$ fly $+a/+b$ produces the following offspring:
$++/ab = 9$
$ab/ab = 9$
$+b/ab = 41$
$a+/ab = 41$
What is the distance between the linked genes?

When a test cross is performed between $F_1$ hybrid flies,the parental type offspring are produced in much higher frequency than the recombinant type offspring. What does this indicate?