Round (R/R or R/r) and wrinkled (r/r) peas are present in a pod of a selfed heterozygous plant (R/r). The phenotypic ratio in this pod happens to be precisely the 3 :1 ratio expected on average in the progeny of this selfing. (Molecular studies have shown that the wrinkled allele used by Mendel is produced by the insertion of a segment of mobile DNA into the gene; see Chapter 15.)
[Madan K. Bhattacharyya.]