In rabbits the gene for grey fur (G) is dominant over that of black fur (g). In a litter, if 50% rabits are grey, the possible parental cross combination is

Options

A. `GG xx Gg`
B. `GG xx GG`
C. `gg xx gg`
D. `Gg xx gg`

Answer

To solve the problem, we need to determine the possible parental cross combination that results in a litter of rabbits where 50% are grey and 50% are black.

### Step-by-Step Solution:

1. **Identify the Genes**:
- The gene for grey fur is dominant (G).
- The gene for black fur is recessive (g).

2. **Understand the Ratios**:
- The problem states that 50% of the rabbits are grey and 50% are black. This indicates a 1:1 phenotypic ratio.

3. **Determine the Parental Genotypes**:
- For a 1:1 ratio to occur, one of the parents must be homozygous recessive (gg), while the other parent can either be homozygous dominant (GG) or heterozygous (Gg).

4. **Test Cross Concept**:
- A test cross is performed by crossing an individual with a homozygous recessive individual. This is used to determine the genotype of the individual with the dominant phenotype.

5. **Possible Parental Combinations**:
- If we cross a homozygous dominant (GG) with a homozygous recessive (gg), all offspring will be heterozygous (Gg) and thus grey.
- If we cross a heterozygous (Gg) with a homozygous recessive (gg), we will get a 1:1 ratio:
- 50% Gg (grey)
- 50% gg (black)

6. **Conclusion**:
- The only combination that gives a 1:1 ratio of grey to black is when one parent is heterozygous (Gg) and the other is homozygous recessive (gg).

### Final Answer:
The possible parental cross combination is **Gg x gg**.

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