USA Biology Olympiad (USABO) Practice Exam

What floral pattern results from a mutation in Gene C that prevents its expression?

1. Petals-Sepals-Carpels-Stamen

2. Sepals-Petals-Petals-Sepals

3. Carpels-Stamen-Sepals-Petals

4. Semi-Petals-Sepals-Sepals

The correct answer is: Sepals-Petals-Petals-Sepals

The correct answer reflects the principles of the ABC model of flower development, which describes how genes regulate the formation of floral organs. In this model, Gene A is responsible for the development of sepals, Gene B for the formation of petals and stamens, and Gene C for carpels. When there is a mutation in Gene C that prevents its expression, the functions that normally direct the formation of carpels are disrupted. Consequently, the pathways that are activated would lead to a situation where genes that typically produce other floral parts (such as Gene A for sepals and Gene B for petals) take precedence in positions normally assigned to carpels. This results in variable expression of the flower's structural components, where the absence of Gene C leads to an overrepresentation of petals and sepals in places where carpels should have developed, creating a pattern of sepals and petals in various configurations. Therefore, the resulting floral pattern indicated in the correct answer is reflective of this misregulation, showing an increase in petals where there would typically be carpels. Each of the other potential choices presents floral arrangements that incorporate carpels, which would not occur if Gene C is not expressed. This understanding is pivotal in grasping the genetic regulation of floral architecture, which