Genetic crossover...
It is possible to breed a trait into a new genetic line, although it takes quite a bit of time. During meiosis, genes can swap between the two chromosones. That is the key to successful incorporation.
If one breeds MS with Mundane, the offspring will all contain one MS chromosone and one Mundane chromosone. Interbreeding the second generation can produce pure MS offspring, even though they retain some of the other mundane silkworm characteristics.
Personally I doubt the 'sloppy husbandry' explanations for the genetic drift. We must keep in mind that we are adding new genes into the composition of the silkworms and not subtracting them, but they already have mundane genes to make silk. Thus you must add an inhibitor to suppress the ordinary silk genes from being expressed.
That inhibitor can be lost over generations due to chromosonal crossover. In that case, an nth generation offspring will suddenly start to produce a silk that is a blend of DS and mundane.
Given the information we have at hand, we cannot tell for certain which explanation is the case. However, I believe that the genetic drift is not simply due to mixing different strains together. It is likely a more difficult problem.
Over many generations, the likelihood for drift can probably be reduced to a small value. The only good news is that KBLB appears to be able to test the silkworms now before mating and can do a better job of selective breeding than it could beore.
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