Who coined the term linkage ?

Thomas Hunt Morgan.

Morgan introduced the term "linkage" after conducting dihybrid cross experiments involving the fruit fly Drosophila. During these crosses, he noted that two genes did not always assort independently as described by Mendel's laws. His observations revealed that when two genes are located on the same chromosome, the occurrence of the parental combination is notably higher in the subsequent generation compared to the non-parental combination.

In genetics, the linkage is the tendency of genes to get inherited together during meiosis of the sexual reproduction, when the genes are located closely on a chromosome.

Morgan and his group 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 per cent recombination while white and miniature wing showed 37.2 per cent recombination.