Ratio between completely homozygous dominant and homozygous recessive individuals of a dihybrid cross is

Options

A. `1:1`
B. `2:2`
C. `6:10`
D. `10:6`

Answer

To solve the question regarding the ratio between completely homozygous dominant and homozygous recessive individuals in a dihybrid cross, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Dihybrid Cross**:
A dihybrid cross involves two traits, each represented by two alleles. For example, consider traits A (dominant) and a (recessive) for one gene, and B (dominant) and b (recessive) for another gene.

2. **Determine the Genotypes**:
In a dihybrid cross, the parental generation (P) can be represented as:
- Homozygous dominant for both traits: AABB
- Homozygous recessive for both traits: aabb

3. **F1 Generation**:
When these two parents are crossed (AABB x aabb), all offspring in the F1 generation will be heterozygous (AaBb).

4. **F2 Generation**:
When the F1 generation (AaBb) is self-crossed (AaBb x AaBb), the F2 generation will exhibit a phenotypic ratio of 9:3:3:1. However, we are interested in the genotypic ratio.

5. **Genotypic Ratio in F2**:
The genotypic ratio in the F2 generation can be calculated using a Punnett square or by applying the principles of Mendelian genetics. The resulting genotypes will be:
- 1 AABB (homozygous dominant)
- 2 AABb (heterozygous)
- 2 AaBB (heterozygous)
- 4 AaBb (heterozygous)
- 1 AAbb (homozygous recessive)
- 2 Aabb (heterozygous)
- 1 aaBB (homozygous recessive)
- 2 aaBb (heterozygous)
- 1 aabb (homozygous recessive)

6. **Count the Homozygous Individuals**:
From the above genotypes, we can see:
- Completely homozygous dominant (AABB): 1
- Completely homozygous recessive (aabb): 1

7. **Calculate the Ratio**:
Therefore, the ratio of completely homozygous dominant (AABB) to completely homozygous recessive (aabb) individuals in the F2 generation is:
- 1 (AABB) : 1 (aabb)

### Final Answer:
The ratio between completely homozygous dominant and homozygous recessive individuals of a dihybrid cross is **1:1**.

---