Double-flower forms often arise when some or all of the
stamens in a flower are replaced by
petals. These types of mutations, where one organ in a developing organism is replaced with another, are known as
homeotic mutations. They are usually
recessive, although the double flower mutation in
carnations exhibits
incomplete dominance. In
Arabidopsis, which has been used as a model for understanding flower development, the double-flower gene AGAMOUS encodes a protein responsible for tissue specification of stamen and
carpel flower segments. When both copies of the gene are deleted or otherwise damaged, developing flowers lack the signals to form stamen and carpel segments. Regions which would have formed stamens instead default to petals and the carpel region develops into a new flower, resulting in a recursive
sepal-petal-petal pattern. Because no stamens and carpels form, the plants have no reproductive organs and are sexually sterile. Mutations affecting flower morphology in
Arabidopsis can be described by the
ABC model of flower development. In this model, genes involved in flower formation belong to one of three classes of genes: A class genes which affect sepal and petal formation, B class genes which affect petal and stamen formation, and C class genes which affect stamen and carpel formation. These genes are expressed in certain regions of the developing flower and are responsible for development of organs in those regions. Agamous is a C class gene, a
transcription factor responsible for activating genes involved in stamen and carpel development. == Gallery ==