Mendelian ratio `9:3:3:1` is due to :-

Options

A. Law of segregation
B. Law of purity of gametes
C. Law of independent assortment
D. Law of unit characters

Answer

To determine the reason for the Mendelian ratio of 9:3:3:1, we need to analyze the options provided and understand the genetic principles involved.

### Step-by-Step Solution:

1. **Understanding the Ratio**: The Mendelian ratio of 9:3:3:1 typically arises from a dihybrid cross, which involves two traits that are being studied simultaneously. This ratio indicates the phenotypic outcomes of offspring when two heterozygous parents are crossed.

2. **Identifying the Laws of Inheritance**:
- **Law of Segregation**: This law states that during gamete formation, the alleles for a trait segregate from each other so that each gamete carries only one allele for each trait. This law explains the separation of alleles but does not directly account for the 9:3:3:1 ratio.

- **Law of Purity of Gametes**: This is essentially another way of describing the law of segregation, emphasizing that gametes are pure and carry only one allele for each trait. Again, this does not explain the 9:3:3:1 ratio.

- **Law of Independent Assortment**: This law states that alleles of different genes assort independently of one another during gamete formation. This means that the inheritance of one trait will not affect the inheritance of another trait. This law directly leads to the 9:3:3:1 ratio observed in dihybrid crosses.

- **Law of Unit Characters**: This law states that each trait is controlled by discrete units (now known as genes). While this law is fundamental to genetics, it does not specifically explain the 9:3:3:1 ratio.

3. **Conclusion**: The 9:3:3:1 ratio is specifically due to the **Law of Independent Assortment**, as it explains how two traits can segregate independently during gamete formation, leading to the variety of combinations seen in the offspring.

### Final Answer:
The Mendelian ratio of 9:3:3:1 is due to the **Law of Independent Assortment**.

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