One of the traits studied by Mendel in pea (Pisum sativum L.) was the wrinkled-seededphenotype, and the molecular basis for a mutation underlying this phenotype was discovered inthe 1990s. Although the starch-branching enzyme gene mutation identified at the genetic locus ris most likely to be that in seeds available to Mendel in the mid-1800s, it has remained an openquestion as to whether or not additional natural mutations in this gene exist within Pisum germplasmcollections. Here, we explore this question and show that all but two wrinkled-seeded variants inone such collection correspond to either the mutant allele described previously for the r locus or amutation at a second genetic locus, rb, affecting the gene encoding the large subunit of adenosinediphosphoglucose (ADP-glucose) pyrophosphorylase; the molecular basis for the rb mutation isdescribed here. The genetic basis for the phenotype of one (JI 2110) of the two lines which are neitherr nor rb has been studied in crosses with a round-seeded variant (JI 281), for which extensive geneticmarker data were expected. In marked contrast to the trait studied by Mendel and the rb phenotype;the data suggest that the wrinkled-seeded phenotype in JI 2110 is maternally determined, controlledby two genetic loci, and the extent to which it is manifested is very sensitive to the environment.Metabolite analysis of the cotyledons of JI 2110 revealed a profile for sucrose and sucrose-derivedcompounds that was more similar to that of wild-type round-seeded, than that of wrinkled-seeded r,pea lines. However, the metabolite profile of the seed coat (testa) of JI 2110 was distinct from that ofother round-seeded genotypes tested which, together with analysis of recombinant inbred progenylines, suggests an explanation for the seed phenotype.