Find out the `F_(2)` phenotypic ratio, when a mulatto individual is cross ed to its homozygous dominant individual. (Given that skin colour is controlled by three gene pairs )

Options

A. `1:4:6:4:1`
B. `1:1:1:1`
C. `1:3:3:1`
D. `3:1`

Answer

To find the F2 phenotypic ratio when a mulatto individual is crossed with a homozygous dominant individual, we need to understand the genetics involved, particularly in the context of polygenic inheritance. Here’s a step-by-step solution:

### Step 1: Define the Genotypes
- A **mulatto individual** is heterozygous for all three gene pairs controlling skin color. We can represent this as **AaBbCc**, where:
- A, B, and C are the dominant alleles for darker skin color.
- a, b, and c are the recessive alleles for lighter skin color.

- A **homozygous dominant individual** will have the genotype **AABBCC**.

### Step 2: Set Up the Cross
We will cross the two individuals:
- Parent 1 (Mulatto): AaBbCc
- Parent 2 (Homozygous dominant): AABBCC

### Step 3: Determine the Gametes
- The gametes produced by the mulatto individual (AaBbCc) can be:
- ABC, ABc, AbC, Abc, aBC, aBc, abC, abc (8 combinations)

- The homozygous dominant individual (AABBCC) can only produce one type of gamete:
- ABC

### Step 4: Perform the Cross
Using a Punnett square, we can combine the gametes:
- Since the homozygous dominant parent can only produce ABC, we will combine it with each of the gametes from the mulatto parent.

### Step 5: Analyze the Offspring
The offspring from the cross will have the following genotypes:
- From gamete ABC (from homozygous parent) and each of the gametes from the mulatto parent:
- ABC (from AaBbCc) + ABC (from AABBCC) = AABBCc (Darker skin)
- ABC (from AaBbCc) + ABC (from AABBCC) = AABbCc (Darker skin)
- ABC (from AaBbCc) + ABC (from AABBCC) = AaBBCC (Darker skin)
- ABC (from AaBbCc) + ABC (from AABBCC) = AaBbCC (Darker skin)

All offspring will exhibit a darker skin phenotype since they all carry at least one dominant allele from the homozygous parent.

### Step 6: Determine the F2 Phenotypic Ratio
Since all offspring from this cross will have at least one dominant allele from the homozygous parent, they will all display the dominant phenotype (darker skin). Therefore, the phenotypic ratio will be:
- **Phenotypic Ratio: 100% Dark Skin (Darker Skin Color)**

### Conclusion
The F2 phenotypic ratio when a mulatto individual is crossed with a homozygous dominant individual is **1:0** (or simply 100% darker skin).

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