The ratio of `9 : 7` is due to:

Options

A. Supplementary genes
B. Lethal genes
C. Complementary genes
D. Epistatic genes

Answer

To determine the cause of the ratio of 9:7, we need to understand the genetic interactions involved. Here’s a step-by-step solution:

### Step 1: Identify the Genetic Interaction
The ratio of 9:7 is indicative of a specific type of gene interaction. In genetics, different types of gene interactions can lead to various phenotypic ratios in offspring.

**Hint:** Think about the types of gene interactions that can lead to modified Mendelian ratios.

### Step 2: Understand Complementary Genes
Complementary genes are non-allelic genes that work together to produce a particular phenotype. When both dominant alleles are present, they produce a new phenotype that is different from the phenotypes produced by each gene alone.

**Hint:** Recall that complementary genes require the presence of both dominant alleles to express a certain trait.

### Step 3: Analyze the Cross
In the scenario described, a cross between two white-flowered varieties of pea plants resulted in a new phenotype (purple flowers) when both dominant alleles were present. This indicates that both genes contribute to the expression of the flower color.

**Hint:** Consider how the presence of both dominant alleles leads to a different phenotype compared to when only one is present.

### Step 4: Determine the Phenotypic Ratio
When examining the offspring (F2 generation), the presence of complementary genes leads to a phenotypic ratio of 9:7. This means that out of 16 possible combinations, 9 exhibit one phenotype, while 7 exhibit another.

**Hint:** Remember that the 9:7 ratio arises when both dominant alleles are necessary for the expression of a specific trait.

### Step 5: Conclusion
Based on the analysis, the ratio of 9:7 is due to complementary genes. This is confirmed by the fact that when both dominant alleles are present, they interact to produce a new phenotype.

**Hint:** Revisit the definitions of supplementary, lethal, complementary, and epistatic genes to reinforce your understanding.

### Final Answer
The ratio of 9:7 is due to **complementary genes**.