The wrinkled phenotype of pea seeds is caused by a single genetic locus, which encodes a protein called starch branching enzyme (SBE). SBE is responsible for the synthesis of amylopectin, a branched polysaccharide that makes up the majority of starch in plants. In the wrinkled phenotype, a mutation in the SBE gene results in the production of a non-functional SBE protein, which leads to a decrease in amylopectin synthesis. This results in the accumulation of smaller, more branched starch molecules, which in turn leads to the wrinkled appearance of the pea seeds.