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. 2
B. 0.5
C. 0.25
D. 1

Explanation

To determine the chances of a pregnancy resulting in an affected child when both parents are carriers for thalassemia, an autosomal recessive disorder, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Genotypes of the Parents**:
- Since both parents are carriers for thalassemia, their genotypes can be represented as:
- Father: Rr (where R = normal allele, r = thalassemia allele)
- Mother: Rr

2. **Set Up a Punnett Square**:
- We will create a Punnett square to visualize the possible genetic combinations from the parents.
- The Punnett square will have the alleles from one parent on the top and the alleles from the other parent on the side.

```
R r
----------------
R | RR | Rr |
----------------
r | Rr | rr |
----------------
```

3. **Analyze the Offspring Genotypes**:
- From the Punnett square, we can see the possible genotypes for the offspring:
- RR (normal, not a carrier)
- Rr (carrier, not affected)
- Rr (carrier, not affected)
- rr (affected by thalassemia)

4. **Calculate the Probabilities**:
- Out of the four possible genotypes:
- 1 is RR (normal)
- 2 are Rr (carriers)
- 1 is rr (affected)
- Therefore, the probability of having an affected child (rr) is:
- Probability = Number of affected offspring / Total offspring = 1/4 = 25%

5. **Conclusion**:
- The chances of a pregnancy resulting in an affected child when both parents are carriers for thalassemia is 25%, or 0.25.

### Final Answer:
The chances of pregnancy resulting in an affected child is 25% (0.25).

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