One of Mendel’s seven genes in peas is the I gene on chromosome 1. Peas with at least one dominant allele, I, are yellow at maturity, whereas peas with two recessive genes, ii, are green at maturity. In the paper “Mendel’s Peas and the Nature of the Gene: Genes Code for Proteins and Proteins Determine Phenotype” (Offner, 2011), I described the I gene as coding for PAO, an enzyme involved in the degradation of chlorophyll. It turns out that the situation is more complicated than this.
The I gene codes for a protein called “stay green” (SGR). The function of this protein is not known. It is not PAO, because the gene that codes for PAO has been mapped to pea chromosome 7 (Moffet & Weeden, 2005). For some unknown reason, the reaction catalyzed by PAO does not occur unless the SGR protein is present. Because...