In the cross performed by Punnet and Batson on Latyrus Odoratus the plants with only C_P_ genotype i.e with two dominant allele have purple colour. At molecular level this is because the two dominant alleles encode for two enzymes that are envolved in anthocyanin synthesis. Due to absence of one dominant allele (i.e. genotype C_pp or ccP_) the anthocyanin pigment is not produced and the flower is white. I was reading Snustard and Simmon's Principle of genetics and it said that the reccessive alleles i.e ccP_ or C_pp have an epistatic effect on the dominant alleles. It didn't make sense to me. I mean at molecular level the recessive alleles(i.e. cc and pp) have no role in supressing the expression of dominant allele of another gene, isn't it?
Yes, in this case the recessive alleles of one gene do not suppress the dominant alleles directly. However, expression of the gene is not always just production of protein. It is rather the production of functional protein. The dominant allele still produces the normal functional enzyme. However, the enzyme cannot perform its function. If you take into account the enzyme kinetics, you remember that enzyme is inhibited by the accumulated product. Let’s assume that genes C and P encode for enzymes C and P respectively, and product of enzyme C is used by enzyme P. If enzyme P is not normal because of the recessive alleles pp, the product of enzyme C will accumulate and inhibit it. Thus, the enzyme is non-functional, and there is indirect suppression. If enzyme C is not normal because of alleles cc, its product will not be produced, resulting in lack of substrate for enzyme P. Lack of substrate is associated with inactivity of the enzyme P. Thus, in this case the indirect suppression is evident again.