Cystic fibrosis is an autosomal recessive disorder. Consider a cross between two carrier parents, each with genotype Ff.

Options

A. ff
B. FF
C. ffff
D. FfFf

Answer

To solve the problem regarding the inheritance of cystic fibrosis through a cross between two carrier parents (genotype Ff), we can follow these steps:

### Step 1: Understand the Genotypes
- Cystic fibrosis is caused by a recessive allele (f).
- The genotype of the carrier parents is Ff, where:
- F = normal allele (dominant)
- f = cystic fibrosis allele (recessive)

### Step 2: Set Up the Punnett Square
- We will create a Punnett square to visualize the possible genotypes of the offspring.
- The alleles from one parent (Ff) will be placed on the top, and the alleles from the other parent (Ff) will be placed on the side.

```
F f
------------
F | FF | Ff |
------------
f | Ff | ff |
------------
```

### Step 3: Analyze the Punnett Square
- From the Punnett square, we can see the possible genotypes of the offspring:
- **FF** (homozygous dominant) - normal phenotype
- **Ff** (heterozygous) - carrier, normal phenotype
- **Ff** (heterozygous) - carrier, normal phenotype
- **ff** (homozygous recessive) - affected by cystic fibrosis

### Step 4: Determine the Ratios
- The genotypic ratio from the Punnett square is:
- 1 FF : 2 Ff : 1 ff
- The phenotypic ratio is:
- 3 normal (FF and Ff) : 1 affected (ff)

### Conclusion
- Therefore, the probability of an offspring being affected by cystic fibrosis (genotype ff) is 1 out of 4, or 25%.