The explanation of Bateson for coupling and repulsion was not very satisfactory. It was T.H. Morgan (1910), who was working on Drosophila together with his associates A.H. Sturtevant, H.J. Muller and C.B. Bridges, offered a plausible reason for the phenomenon of coupling and repulsion.
According to Morgan, coupling and repulsion are but two facets of the same phenomenon.
He further stated that pairs of genes from homozygous parents tend to enter the same gamete, while the same genes from heterozygous parents tend to enter different gametes and remain apart.
The tendency of the genes to retain their original combination is due to their location on the same chromosome another significant aspect of linkage underscored by Morgan was that the strength of linkage (tendency to go together) depends on the distance between the linked genes, on the chromosome.
The closer the genes, the stronger will be the linkage, and the farther the genes, the weaker will be the linkage.
Tendency to remain together will be more if the linkage is stronger and the tendency to segregate more will be there if the linkage is weak. Morgan and Castle formulated the chromosome theory of linkage.
The main features of this theory are:
1. Genes showing linkage are on the same chromosome.
2. Genes are arranged on the chromosome in a linear fashion. Linkage of genes is also linear.
3. The distance between the genes determines the strength of the linkage. Strong linkage indicates proximity of the genes, while weak linkage indicates that they are farther apart.
4. Linked genes will remain together during inheritance.
Cytological studies have given the physical proof of linkage. Linkage of Genes as observed by the inheritance of traits has helped cytologists to map the chromosomes.