Question

Give reasons? Morgan and his group knew that the genes were located on the X choromosome and saw quickly that
When the two genes in a dihybrid cross were situated on the same chromosome, the proportain of parental gene combinations were much higher than the non-parental type.

Answer 1

When two genes are on the same chromosome and are located close to one another, they are said to be linked genes. When genes are linked recombination does not occur between them and they are inherited together. In a dihybrid cross involving linked genes the gametes formed are only of the parental type (as seen in the image) and therefore, instead of the normal 9:3:3:1 ratio of offsprings in the F2 generation (that is seen due to independent assortment) an F2 ratio of 3:1 is seen, with no recombinant types. Even if recombinants are seen they are very few in number (seen in cases of incomplete linkage).

So, the proportion of parental gene combinations can be said to be much higher.