Describe linkage and recombination in detail.

(N/A) Morgan and his group observed in $Drosophila$ that when two genes in a dihybrid cross were situated on the same chromosome,the proportion of parental gene combinations was much higher than the non-parental type.
They attributed this to the physical association of the two genes and coined the term 'linkage' to describe this physical association of genes on a chromosome,and the term 'recombination' to describe the generation of non-parental gene combinations.
Thus,linkage is a phenomenon of genetic inheritance in which genes on a particular chromosome show a tendency to be inherited together.
Morgan and his group also found that even when genes were grouped on the same chromosome,some genes were tightly linked; i.e.,linkage is stronger between two genes if the frequency of recombination is low.
Conversely,the frequency of recombination is higher if genes are loosely linked; i.e.,linkage is weak between two genes.
Recombination of linked genes occurs by crossing over (the exchange of corresponding parts between the chromatids of homologous chromosomes).
All the genes linked together on a single chromosome constitute a linkage group.
The number of linkage groups in an organism is equal to its haploid number of chromosomes.
This hypothesis was proved by $T.H.$ Morgan through his experiments on $Drosophila$.
Morgan and his group hybridized yellow-bodied,white-eyed females with brown-bodied,red-eyed males (wild type) and intercrossed their $F_1$ progeny (cross-$A$).
It was observed that the two genes did not segregate independently of each other,and the $F_2$ ratios deviated significantly from the $9:3:3:1$ ratio.
In the $F_2$ generation,parental combinations were $98.7\%$ and recombinants were $1.3\%$.
In another cross (cross-$B$),between a white-bodied fly with miniature wings and a male fly with a yellow body and normal wings,parental combinations were $62.8\%$ and recombinants were $37.2\%$ in the $F_2$ generation.
Thus,it was proved from the crosses that the linkage between genes for yellow body and white eyes is stronger than the linkage between the white body and miniature wings.
His student,Alfred Sturtevant,used the frequency of recombination between gene pairs on the same chromosome as a measure of the distance between genes and 'mapped' their position on the chromosome.