View Larger Version | The ABC model of floral development emerged in the late 1980s from research on flowering plants with genetic mutations. The model originally described three classes of mutant plants, linking genes categorized as A, B or C to predictable mistakes in flower structure. A flower normally is made up of four concentric layers, or whorls, of parts: sepals form the outermost whorl, then petals, stamens and carpels. In plants without functioning A, B or C class genes, floral organs developed in the wrong whorl or didn’t develop at all. The revised ABC model, shown above, includes two additional classes of genes (D and E). It shows how the genes work in various combinations to trigger the specialization of tissues into the whorls of a flower. A and E class genes are needed to make sepals; A, B and E for petals. To make stamens, B, C and E genes are active. C and E are required for carpels, and C, D and E for ovules inside the carpels. The model has held up well, but may change as scientists probe floral development in more species. Credit: Illustration by Gina Mikel