Why are the medium protein types so common?
There are two reasons that contribute to making the medium level of protein so common. The first is that plants that are heterozygous at a given gene pair are twice as common as either of the homozygous genotypes (¼ AA : ½ Aa : ¼ aa). The more important reason is that many genotypes can make up an average phenotype. For example, AaBbDdEeFfGGHh, AABBDDeeffGGhh, and aabbddEEFFGGHH all have 8 additive alleles resulting in medium protein levels. In fact there will be 5544 ways to make a plant with the average protein genotype compared to only one way to make a plant with one of the extreme phenotypes. (The 5544 is calculated the same way you would predict the probability of having a 12 child family with all girls compared to one with 6 boys and six girls.) Therefore, the average outcome is very common and the extreme outcome is very rare.