The genetics for undershot jaw appear to be multiple and depending on how many line up depends on the severity of the length. I think with that being said you can have a male that has a lot of genes for a shorter jaw and appear normal but when bred to a female with predominance of genes for undershot might turn out that the litter are right in the middle with their bite since they blended their traits. Leg length is the same, not just simple recessive or dominance; long legs are a combination of recessives, then middle leg length is a combination of recessives and dominant genes, then short legged individuals are dominant genes showing the short legged phenotype. The thing that complicates this scenario is when you get individuals with hybrid vigor that appear larger but in fact are not the long legged recessive types and end up throwing whatever the combinations the parents had given the individual when they are bred.

More things to watch for when putting the punnet square together. I think the best bet is to learn what to look for and then when breeding keep track of the results. This sounds complicated and can be but with a little effort and good notes the puzzle starts to form a picture. Another example is when you have a genius and he/she breeds outside the family....the genius is lost thus we realize this trait is recessive even though it is polygenic. Individuals born to the genius type more than likely are very smart thus have a number of the recessives but not enough to put them in that category. The daughter or son were to go back to the parent then their might be a good chance of producing Genius type children. This could be done multiple times and lock in the trait if the resulting progeny were viable and able to reproduce.