Gene Duplication and Fixed Heterozygosity for Alcohol Dehydrogenase in the Diploid Plant Clarkia franciscana

All individuals of the diploid plant Clarkia franciscana, an annual species native to California, have duplicated genes for the dimeric enzyme alcohol dehydrogenase. Since each gene specifies a polypeptide with a distinct charge, the species has a fixed heterozygous phenotype consisting of three isozyme variants. The enzyme is coded by a single gene in closely related species. Clarkia franciscana is highly self-pollinated and monomorphic at all but one of its loci that have been examined. Consequently, the duplication provides enzyme variability in the absence of genetic variability.