Possible Hypothesis: Is it a lack of dominance?
One potential explanation for this result would be that protein percentage in the seed is a trait controlled by alleles that have a lack of dominance interaction.
The P1 allele contributes to higher protein but has a lack of dominance over the P2 allele controlling seed protein. Therefore, P1P2 plant produces a level of protein that is intermediate or between the P1P1 and P2P2 levels. One way to test this hypothesis would be to further track the inheritance of seed protein levels in this family, as we will see on the next page.