Linked genes are present on

Define/Explain: Linkage.

$A$ 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?

Briefly mention the contribution of $T.H.$ Morgan in genetics.

Who put forward the "Theory of linkage"?

There are three genes $a, b,$ and $c$. The percentage of crossing over between $a$ and $b$ is $20\%$,between $b$ and $c$ is $28\%$,and between $a$ and $c$ is $8\%$. What is the sequence of these genes on the chromosome?