Pre AP Biology -GEN 3.1 RNA Structure- FRQ Exam Style Questions -New Syllabus

The three genes are expressed in different whorls of the flower to provide the identity of sepals, petals, anthers, and carpels. Protein $A$ is essential for the identity of sepals and petals; if the gene for protein $A$ is disabled, the resulting mutants can form only the reproductive structures, specifically the anthers and carpels of the flower.

The $ABC$ model of flower development is a genetic framework explaining how specialized organ identity genes (Class $A$, Class $B$, and Class $C$) coordinate gene expression to specify the four whorls of a flower. In a normal developmental path :

• Whorl 1: Only $A$ genes are active, producing sepals .
• Whorl 2: $A$ and $B$ genes work together to produce petals .
• Whorl 3: $B$ and $C$ genes work together to produce stamens (anthers) .
• Whorl 4: Only $C$ genes are active, producing carpels .

A mutation in these genes represents a heritable change to the DNA sequence that disrupts normal protein function. Because $A$ and $C$ genes are mutually antagonistic, disabling the $A$ gene allows the $C$ gene to be expressed in all whorls.

If the gene for protein $A$ is disabled, the phenotype changes because the instructions for making sepals and petals are lost. The outer whorls, now influenced by $C$ genes alone or $B$ and $C$ together, will transform into reproductive organs. Consequently, the flower will consist only of anthers and carpels, illustrating how specific genotypes determine phenotypes in complex biological systems.