If both parents are carriers for Thalassaemia, which is an autosomal recessive disorder, what are the chances of pregnancy resulting in an affected child?

Options

A. `100%`
B. No chance
C. `50%`
D. `25%`

Answer

To determine the chances of a pregnancy resulting in an affected child when both parents are carriers for Thalassemia, we can use a Punnett square to analyze the genetic combinations. Thalassemia is an autosomal recessive disorder, which means that a child must inherit two copies of the recessive allele (denoted as 'r') to be affected.

### Step-by-Step Solution:

1. **Identify the Genotypes of the Parents**:
- Since both parents are carriers for Thalassemia, their genotypes are Rr (where 'R' is the normal allele and 'r' is the thalassemia allele).

2. **Set Up the Punnett Square**:
- Create a 2x2 Punnett square to visualize the genetic combinations from the two parents.
- The alleles from one parent (Rr) will be placed on the top, and the alleles from the other parent (Rr) will be placed on the side.

```
R r
__________
R | RR | Rr |
|______|______|
r | Rr | rr |
|______|______|
```

3. **Fill in the Punnett Square**:
- The combinations from the Punnett square will be:
- RR (normal, not a carrier)
- Rr (carrier, normal phenotype)
- Rr (carrier, normal phenotype)
- rr (affected by Thalassemia)

4. **Analyze the Results**:
- From the Punnett square, we can see the possible genotypes of the offspring:
- 1 RR (25%)
- 2 Rr (50%)
- 1 rr (25%)
- The only genotype that results in an affected child is 'rr'.

5. **Calculate the Probability**:
- The probability of having an affected child (genotype 'rr') is 1 out of 4, or 25%.

### Conclusion:
The chances of a pregnancy resulting in an affected child when both parents are carriers for Thalassemia is **25%**.