What concept did Morgan introduce to explain the physical association of genes on a chromosome?

Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked. When the 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 due to the physical association or linkage 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.