Does the Lack of Dominance Hypothesis fit?
It is difficult to explain the observed F2 phenotype distribution based on the single gene pair hypothesis. There are several observations that do not fit.
1) There are dozens of different protein level phenotypes observed in the F2, not just three. The variation observed is continuous from low to high rather than discrete with F2s falling into clear categories of seed protein level.
2) The distribution of the protein phenotypes shows that the F2 seeds with an intermediate protein are very common while the high and low extremes are very rare. This distribution does not fit any ratio we have encountered when one gene pair (1:2:1 or 3:1) or even two gene pairs (9:3:4, 12:3:1, 9:3:3:1) control a single trait.
3) There are a few individuals that have a protein level lower than the lowest parent and a few that have higher protein than the highest parent. This would not be possible if P1P1, P1P2, P2P2 were the only possible genotypes for seed protein control.