ABC model

Homeiotic genes -It regulates the develpoment of anatomical structures in plants. The flowers of Arabidopsis (Brassicaceae family) consist of 4 concentric whorls .

Whorl 1=sepal
Whorl 2=petal
Whorl 3=stamen
Whorl 4=carpel

The expression of homeotic genes in 4 whorls results in production of transcription factors which control the phenotype of organs. Three classes of homeotic genes A,B and C determine organ identity by their expression in different whorls of cells.That is called ABC model.

In wild type flower (not mutant) -
 Whorl-1 (sepals) by the product of A genes (Apetala 1 and apetala 2)
 Whorl-2 (petals) by the product of A and B genes (apetala 3 and pistillata)
 Whorl-3 (stamens) by the product of B and C genes(agamous)
 Whorl 4 (carpels) by the action of C genes alone.

All except the Apetala 2 protein contain the same DNA- binding domain, the MADS box.
In mutant forms the role of these genes are deduced by inducing mutation in each of these genes.A and C genes are equally dominant .Mutation in A gene makes C gene more active. B gene always express in association with A and C genes.

(ABC model and changes associted with whorl formation due to mutation in A,B and C gene)

Image source-http://www.biologyexams4u.com/2015/03/abc-model-of-flower-development-in.html#.V0PPJpF97Dd

Q.Following are certain statements regarding the activities of homeiotic genes of classes A,B and C involved in floral oran identity-
a.Activity of A alone specifies sepals
b.Activity of B alone specifies petals
c.Activities of B and C form stamens
d.Activity of C alone specifies carpels
 Which one of the following combinations of above statements is correct?
1.A,B, and C                    3.A,B and D
2.B,C and D                     4.A,C and D

Ans. 4(A,C and D)

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