Or does it mean that a mutation has occured in the fragment which does not allow the cDNA to hybridize to the actual DNA, so therefore no band is shown. If the fragment is actually missing, then I don't understand how an entire exon can be deleted off a chromosome. Next, I'm wondering how specific you want us to be on this or whether I'm digging too deep. Yes indeed, if a band is missing, it is deleted. Deletions are a common form of mutations and involve two breaks in the DNA so the piece between them iis missing.
Deletions can occur in different parts of the gene and can be almost any size.
I'm really confused about what these two problems are asking. The book has a good discussion of Rb in Chapter 15 which might help you make more sense out of problem 2. Treat problem 1 as a problem in logic. It doesn't make any difference what the function of the genes actually is. Think about what it means for the proteins encoded by genes to function in a pathway, based on the many examples of pathways we have seen in class.