T.H. Morgan carried out several dihybrid crosses in Drosophila melanogaster and studied genes that were sex-linked. Based on his studies, he coined the terms linkage and recombination. Linkage describes the physical association of two genes on a chromosome while recombination is the generation of non-parental gene combinations.
He designed three crosses;
Cross 1- Male with white eyes X female with red eyes
F1= only red-eyed flies
F2= all females flies were red-eyed while 50% male was red-eyed and the other 50% were white-eyed.
Cross-2- Red-eyed females of F1 X white-eyed male
Result- red and white-eyed traits appear in both males and females in equal proportion.
Cross-3: White-eyed females X red-eyed males
Result = Criss cross inheritance.
From all these, he concluded that the eye colour gene is linked to X-chromosome.