Linkage decreases the frequency of

$A$ : Crossing over is exchange of genetic material between non-homologous chromosomes.
$R$ : It produces new linkages.

Frequency of recombination between gene pairs on same chromosome as a measure of the distance between genes to map their position on chromosome, was used for the first time by

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

Crossing over causes