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 ii peas do not make the SGR protein, they do not degrade their chlorophyll and they have a green phenotype. I thank Stefan Hörtensteiner for pointing out this correction. Further details of what is known about SGR function can be found in Hörtensteiner (2009).
So now, nearly 150 years after Mendel published his work, we can say that two of his genes are well understood at the biochemical level (tall/short plants; round/wrinkled peas), two are partially understood (red/white flowers; yellow/green peas), and three are not understood at all (axial/terminal flowers; inflated/constricted pods; green/yellow pods).