Not a stupid question at all actually.
Yes, spontaneous mutations usually arise in the germ cells before an embryo is created. If a mutation arose in the embryo, say at the two cell stage, it would occur in only one of the cells (not both) and the person who developed from that embryo would be mosiac, ie 50% of their cells would have 1 CFTR mutation and 50% of their cells would have zero mutations. If the embryo had already inherited a mutation from a parent and a spontaneous mutation arose at the two cell stage, whether or not they had symptoms would be dependant on the genotype of the pertinant tissues. For instance, the lungs and pancreas might be just carriers where other tissues might have have the cf geneotype. Another possibility is that the lungs and pancreas could be mosiacs but enough functional CFTR would be present (in the cells that are just carriers) to prevent any serious symptoms from developing (mosiac tissue is probably most likely option here). It is equally possible that the mosiac tissues could be affected with disease too. If you assume that both mutations completely knocked out CFTR function then the mosiac tissues would have 25% CFTR function which may be enough to keep a person healthy or could be not enough for disease to occur. And finally it is possible that the lungs and pancreas could have the CF genotype and the person could have classic cf. All of these things are pretty unlikely though.
Remember that it takes a huge loss of functional CFTR before disease occurs. Delf508 carriers have only 50% functional CFTR and most of them are healthy, normal individuals . I think i've heard that people can have only 10% CFTR function and still be healthy. (this most likely varies somewhat among indivduals though)
I guessing most spontaneous mutations that are relevant clinically occur in the germ cells becuase the embryo divides rapidly so there is a very small time frame for which a mutation could occur that would go on to be present in large % of the indiviudual's cells. Dominent mutations (cf is recessive) are probably more likely to cause problems in mosiac individuals too. Embryos with issues replicating DNA usually spontaneously abort too which is probably another reason why spontaneous mutations usually occur in germ cells.
Must get back to writing dissertation. keep editing for clarification. posting is addicting. must stop.
