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

Options

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

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. Thalassemia is an autosomal recessive disorder, which means that an individual must inherit two copies of the mutated gene (one from each parent) to be affected by the disorder.

### Step-by-Step Solution:

1. **Identify the Genotypes of the Parents**:
- Since both parents are carriers for thalassemia, their genotypes can be represented as "Tt", where "T" is the normal allele and "t" is the allele for thalassemia.

2. **Set Up the Punnett Square**:
- Create a 2x2 Punnett square to show the possible combinations of alleles from the parents.
- The alleles from one parent will be placed on the top of the square, and the alleles from the other parent will be placed on the side.

```
T t
------------
T | TT | Tt |
------------
t | Tt | tt |
```

3. **Fill in the Punnett Square**:
- Fill in the squares by combining the alleles from the top and the side.
- The combinations will be:
- TT (homozygous normal)
- Tt (heterozygous carrier)
- Tt (heterozygous carrier)
- tt (homozygous affected)

4. **Analyze the Results**:
- From the Punnett square, we see the following genotypic ratios:
- 1 TT (normal)
- 2 Tt (carriers)
- 1 tt (affected)
- This means there is a 1 in 4 chance (or 25%) of having a child that is affected by thalassemia (tt).

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