In mice Y is the dominant allele for yellow fur and y is the recessive allele for grey fur. Since Y is lethal when homozygous, the result of cross `Yy xx Yy` will be

Options

A. 3 yellow : 1 grey
B. 2 yellow : 1 grey
C. 1 yellow : 1 grey
D. 1 yellow : 2 grey

Explanation

To solve the problem of the cross between two heterozygous mice (Yy x Yy), we need to follow these steps:

### Step 1: Identify the Alleles
We have two alleles for fur color in mice:
- Y (dominant allele for yellow fur)
- y (recessive allele for grey fur)

### Step 2: Understand the Lethal Condition
The allele Y is lethal when homozygous (YY). This means that any mouse with the genotype YY will not survive.

### Step 3: Set Up the Punnett Square
We will set up a Punnett square to determine the possible genotypes from the cross Yy x Yy.

```
Y y
----------------
Y | YY | Yy |
----------------
y | Yy | yy |
----------------
```

### Step 4: Determine the Genotypes
From the Punnett square, we can see the following genotypes:
- YY (homozygous yellow)
- Yy (heterozygous yellow)
- yy (homozygous grey)

### Step 5: Apply the Lethal Condition
Since YY is lethal, we cannot count it in our final proportions. Therefore, we only consider the surviving genotypes:
- Yy (heterozygous yellow)
- yy (homozygous grey)

### Step 6: Count the Surviving Proportions
The surviving genotypes from the cross are:
- 2 Yy (yellow fur)
- 1 yy (grey fur)

Thus, the ratio of surviving phenotypes is:
- Yellow fur (Yy) : Grey fur (yy) = 2 : 1

### Final Result
The result of the cross Yy x Yy, considering the lethal condition of YY, will yield a phenotypic ratio of **2 yellow : 1 grey**.

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