genetic cross question?

um.. cuz all i know is as u progress in that subject you're in, ur gna get to flies experiments which ur gna require to use more flies to breed. its gna cause flies to look different than the usual like a fly with a black body and white eyes, something like that... maybe u gotta use new flies.. the flies ur using myt not be "virgin" flies anymore... thats why the genes they show are a bit messed up. the question's a bit unclear. id be glad to help cuz i just finished that course last last sem.. repost it and fix ur question a bit. ill try to help.

I agree with the previous poster, I don't think your question is clear but I can attempt to answer it with what you have. There are several genes and related phenotypes that one can study in fruit flies (a lot of the mutants have rather funny names too). But to me, the only way in which you may not get a wild type (in fruit flies or anything for that matter) when you cross two pure breeds (which is what your question implies), is if the mutation in the pure breed non-wild type is dominantly inherited. If that is the case, then having only one mutant allele would result in having a mutant phenotype. Recall that wild type means is the phenotype you would normally see in nature. In most cases, when a mutation occurs, it isn't normally dominantly inherited (indeed, many mutations are masked by heterozygote individuals in the population), but there can be occassions of dominance. Also, let us not forget the prospect of incomplete dominance (if the mutant allele shows up partially), which means the individual would still not be wild type.
Let's say that the phenotype isn't inherited dominantly. If you cross two heterozygotes (breed to the F2 generation; P is the purebreeds, F1 is the first cross you referred to), you will obviously have a 0.25 chance of getting a non-wild type phenotype.
Hope that helps. If not, repost the question, worded differently and we'll see if we can figure something out. It would help if you give a more concrete example of a genotype/phenotype cross that results in non-wild type offspring.
Edit: I'm still unclear on your notes. From what I can understand, your lecture notes are invoking the argument of incomplete dominance at the cellular level (whereby you get half the gene product because only one gene is "normal" and "working."
Your question is not easily answered because I'm not sure what type of homozygous mutant or what the mutant gene phenotype is suppose to look like, and this is important for specifying a mechanism. Is the mutant phenotype considered a "muddled" or toned down version of the wild type or is it a completely different phenotype (like white eyes as opposed to orange or pink eyes) ? If you are talking about something like a white eye mutant (where the mutation results in a phenotype that is completely different than the wild type) then consider this: Like I said, the wild type doesn't necessarily have to be dominant one (and this is not always the case), sometimes, the mutant allele is dominant. This can happen if there is a product of the mutant allele that can override the other allele, mask that other allele or modify the other allele so that only the mutant is expressed. Basically, I'm talking about gene interaction, specifically epistasis (where one gene controls another).