The round seed phenotype is derived from the R allele of the SBE1 gene in Pisum sativum pea plants. This allele codes for a functional starch branching enzyme that converts amylose into amylopectin. Conversely, the wrinkled seed phenotype is derived from the recessive r allele. This allele is a variant of the R allele of the SBE1 gene that codes for a non-functional starch branching enzyme.
Effect of the Alleles
The R allele of the SBE1 gene is comprised of 3550 nucleotides. These nucleotides code for a chain of 960 amino acids that form the starch branching enzyme. The r allele is identical to the R allele with one notable exception – there is an extra segment of 800 nucleotides that has been inserted into the middle, making is nearly 4400 nucleotides in length. This insertion mutation means that the enzyme formed by the resulting amino acid chain is essentially non-functional and unable to convert amylose into amylopectin.
Instead, the presence of the r allele results in excess glucose that combines with fructose to produce sucrose. The additional sucrose means that homozygous rr pea seeds taste sweeter. Any pea plant that has at least one R allele will be able to make functional starch branching enzymes for use within pea plant seed cells. This explains why the R allele is considered dominant over the r allele.