What is a linkage group?

Do you think Mendel's laws of inheritance would have been different if the characters that he chose were located on the same chromosome?

Which traits did Morgan choose for the male and female $Drosophila$ respectively to explain linkage and recombination?

The linkage map of $X$-chromosome of fruit fly has $66$ units with yellow body gene $(y)$ at one end and bobbed hair $(b)$ gene at the other end. The recombination frequency between these two genes ($y$ and $b$) should be

$A$ scientist performed a gene mapping experiment in maize. He mapped genes on chromosomes based on the percentage of crossing over between different genes. One map unit corresponds to $1\%$ crossing over or recombination frequency. Based on the study of crossing over in maize, the scientist observed the following recombination percentages between genes $A, B, C,$ and $D$: $A$ and $D = 10\%$, $A$ and $C = 3\%$, $C$ and $D = 7\%$, $A$ and $B = 5\%$, and $C$ and $B = 8\%$. Based on these observations, determine the correct order of genes $A, B, C,$ and $D$ on the chromosome.

$A$: Morgan's cross $III$ was conducted in $Drosophila$ to locate genes on the chromosome for white eye color.
$R$: The cross was done between red-eyed hybrid female and white-eyed male.