NOTE 21E: A full calculation of the dynamics is complicated, because there will be some association (i.e., some linkage disequilibrium) between the restorer allele and the sterility allele, although this will be weak, because the genes are not linked. There may be a polymorphism if the restorer allele has some cost; in addition, the sterility allele gains some extra advantage by avoiding self-fertilization and hence inbreeding depression. On the other hand, selfing by male-fertile plants may prevent the population becoming extinct. Finally, new restorer alleles and new sterility alleles may evolve, so that there will be a continuing coevolution between these genes. This “arms race” is ultimately caused by a conflict between genes that are inherited in different ways. See Web Notes.