Flower Development :

The molecular control of floral organ identity determination is fairly well understood. In a simple model, three gene activities interact in a combinatorial manner to determine the developmental identities of the organ primordia within the floral meristem. These gene functions are called A, B and C-gene functions. In the first floral whorl only A-genes are expressed, leading to the formation of sepals. In the second whorl both A- and B-genes are expressed, leading to the formation of petals. In the thrid whorl, B and C genes interact to form stamens and in the center of the flower C-genes alone give rise to carpels. The model is based upon studies of homeotic mutants in Arabidopsis thaliana and snapdragon, Antirrhinum majus. For example, in a loss of B-gene function mutant flower we get sepals in the first whorl asu usual, but also in the second whorl (the B-function lost that is needed for petal development). In the third whorl the lack of B function but presence of C-function mimics the fourth whorl, leading to the formation of carpels also in the third whorl. See also The ABC Model of Flower Development.