The coupling and repulsion theory of Bateson and Punnett later on modified in linkage and crossing over by Morgan. Two completely linked gene shows a dihybrid cross was A) \[3:1\] B) \[1:1\] C) \[1:1:1:1\] D) \[9:3:3:1\]
Hint: When the gene resides on the same chromosome pair is termed as linkage and the linkage between the two dominant alleles and two recessive alleles is termed as coupling. Repulsion indicates that dominant alleles are linked with recessive alleles.
Complete answer: Several dihybrid crosses were carried out in drosophila by Morgan to study the sex-linked genes. These crosses were quite similar to the hybrid crosses carried out by Mendel in peas. For example, Morgan hybridized yellow bodied white-eyed females to brown bodied Red-eyed males and intercrossed their F1 progeny. He observed that the two genes did not segregate independently of each other and the F2 ratio deviated very significantly from the\[9:3:3:1\] ratio. Morgan and his group were aware that the genes were located on the X chromosome and noticed quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much more advanced than the non-parental kind. Morgan attributed this is due to the physical association or linkage of the two genes the coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to explain the generation of non-parental gene combinations. He and his group found that two completely linked genes showed a dihybrid ratio of\[1:1:1:1\].
Thus option C is the correct answer.
Note: Morgan and his group also found that even when genes are grouped on the same chromosome, some genes were very tightly linked while others were loosely linked. For example, he found that the white and yellow very very tightly linked and showed only \[1.3\]\[\% \] recombination while the white and miniature wing showed \[37.5\]\[\% \] recombination.