State the conclusions drawn from Morgan's experiments on linkage and recombination.

(A) Morgan conducted several dihybrid crosses in Drosophila to study genes that were sex-linked. These crosses were similar to the dihybrid crosses carried out by Mendel in peas.
Morgan hybridized yellow-bodied,white-eyed females to brown-bodied,red-eyed males and intercrossed their $F_1$ progeny.
He observed that the two genes did not segregate independently of each other and the $F_2$ ratio deviated very significantly from the $9:3:3:1$ ratio (expected when the two genes are independent).
Morgan and his group knew that the genes were located on the $X$ chromosome. They observed 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.
Morgan attributed this to the physical association or linkage of the two genes. He 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.
Morgan and his colleagues also found that even when genes were grouped on the same chromosome,some genes were very tightly linked (showed very low recombination) while others were loosely linked (showed higher recombination).
For example,he found that the genes white and yellow were very tightly linked and showed only $1.3\%$ recombination,while white and miniature wing showed $37.2\%$ recombination,indicating loose linkage.
Morgan's 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.
Genetic maps are now used as a starting point in the sequencing of whole genomes,as was done in the case of the Human Genome Project $(HGP)$.