Why did Mendel face difficulties in explaining linked traits?

Mendel observed that some traits do not assort independently. Later research showed that this is due to .....

When white-eyed and miniature-winged $Drosophila$ $melanogaster$ is crossed with its wild type,it produces which of the following percentages of recombinants (in $\%$)?

What will be the number of linkage groups in maize if it has $10$ pairs of chromosomes?

$A$ : 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.

There are three genes $a, b, c$. 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 genes on the chromosome?