A plant having 24 cm long internodes is crossed with a plant having 12 cm long internodes. The hybrids have 18 cm long internodes due to

Options

A. Multiple allelism
B. Complete dominance
C. Recessive dominance
D. Incomplete dominance

Answer

To solve the problem, we need to analyze the given information about the plants and their internode lengths. Here’s a step-by-step solution:

### Step 1: Understand the Cross
We have two plants:
- Plant A with 24 cm long internodes (let's assume this is the dominant trait).
- Plant B with 12 cm long internodes (let's assume this is the recessive trait).

### Step 2: Analyze the Hybrid Offspring
When these two plants are crossed, the resulting hybrids have internodes measuring 18 cm. This measurement is between the lengths of the two parent plants.

### Step 3: Determine the Type of Inheritance
The key observation here is that the hybrid (18 cm) is not exhibiting the trait of either parent fully (24 cm or 12 cm). Instead, it shows an intermediate trait. This suggests that the inheritance pattern is not one of complete dominance, where one trait completely masks the other.

### Step 4: Eliminate Other Options
- **Complete Dominance**: If complete dominance were at play, the hybrids would show the dominant trait (24 cm), but they do not. Thus, this option is incorrect.
- **Recessive Dominance**: If recessive dominance were the case, the hybrids would show the recessive trait (12 cm), which is also not the case. Therefore, this option is incorrect.
- **Multiple Allelism**: This refers to a situation where multiple alleles control a single trait. However, the problem does not indicate the presence of multiple alleles affecting the internode length. Thus, this option is also incorrect.

### Step 5: Conclude with Incomplete Dominance
Since the hybrid shows an intermediate trait (18 cm), we conclude that the inheritance pattern is one of **incomplete dominance**. In this case, neither parent trait is fully expressed, resulting in a blend of the two traits.

### Final Answer
The hybrids have 18 cm long internodes due to **incomplete dominance**.

---