Dominance or a lack of dominance
In many situations, the different versions of the genes (alleles) that control a grain quality trait will have a lack of dominance. This means that heterozygous individuals will have a phenotype that is different from (normally in between) either of the homozygous genotypes. If an intermediate type is not expressing the grain quality trait to an acceptable level, the seed producer will be required to produce homozygous seed for the producer to grow. This is not an issue for a self-pollinated crop but it would be a concern for someone like Dr. Russell in developing corn hybrids with low seed phosphorous (P) levels. If the alleles that control low seed P have a dominant effect, only one low P parent line would need to be developed. The understanding the plant breeder develops as they work with these traits will thus be critical in helping the seed producer make the right decisions.