Molecular Basis of Dominance
The occurrence of the IT, IR and normal alleles of the ALS enzyme and their impact on plant phenotype provides us with an opportunity to think about dominance and lack of dominance at the molecular level. The IT allele has complete dominance to the normal allele because there is apparently no phenotype difference between homozygous and heterozygous IT plants when they are sprayed with an ALS inhibitor. This suggests that in the heterozygous plants, one copy of the IT allele per cell encodes enough resistant ALS protein to drive the synthesis of sufficient quantities of Val, Leu and Ile for normal growth in the corn plant. However, the ALS enzyme encoded by the normal allele in these heterozygotes can be inhibited by the herbicide and will not contribute to amino acid synthesis.
In contrast, the IR allele has a lack of dominance to the normal allele (IN). Heterozygous plants that have one copy of the IR and one copy of the normal allele per cell do not maintain normal growth after ALS inhibitor treatment. One copy of the IR allele per cell does not encode sufficient copies of a resistant ALS enzyme to support normal growth. Instead, two copies of the IR allele per cell are needed so that all of the ALS enzyme made will not be bound by the herbicide.
The classification of the plant phenotype for characterizing possible genotypes at the ALS locus may depend on environment. In some growing environments it has been noticed that heterozygous IT hybrids will show a slight growth reduction in the sprayed plants compared to unsprayed. The difference is minor because the plants usually grow out of this difference later on and yield reduction is not a factor. This observation does emphasize that phenotype is a result of genotype and environment. Therefore scoring traits in the proper environment will be critical to discovering the nature of gene action.