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

Answer

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

### Step-by-Step Solution:

1. **Understand the Genetic Basis**: Thalassemia is an autosomal recessive disorder. This means that an individual must inherit two copies of the mutated gene (one from each parent) to be affected by the disorder.

2. **Identify Genotypes of Parents**: Since both parents are carriers, their genotypes can be represented as:
- Parent 1: Aa (carrier)
- Parent 2: Aa (carrier)
Here, 'A' represents the normal allele and 'a' represents the thalassemia allele.

3. **Set Up a Punnett Square**: To find the possible genotypes of the offspring, we can set up a Punnett square:
```
A a
-----------------
A | AA | Aa |
-----------------
a | Aa | aa |
-----------------
```
- The rows and columns represent the alleles contributed by each parent.

4. **Determine Offspring Genotypes**: From the Punnett square, we can see the possible combinations of alleles for the offspring:
- AA (normal)
- Aa (carrier)
- Aa (carrier)
- aa (affected)

5. **Count the Outcomes**:
- 1 AA (normal)
- 2 Aa (carriers)
- 1 aa (affected)

6. **Calculate the Probability**: To find the probability of having an affected child (aa), we take the number of affected offspring over the total number of offspring:
- Probability of affected child = Number of affected (aa) / Total outcomes = 1 / 4 = 25%

7. **Conclusion**: Therefore, the chances of a pregnancy resulting in an affected child when both parents are carriers for thalassemia is 25%.

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

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