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

Options

A. 0.5
B. 0.25
C. 1
D. Non chance

Answer

To determine the chances of a pregnant woman resulting in an affected child from parents who are both carriers for thalassemia (an autosomal recessive disorder), we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Genetics of Thalassemia**:
- Thalassemia is an autosomal recessive disorder, meaning that an individual must inherit two recessive alleles (let's denote them as 'a') to express the disease. The normal allele is denoted as 'A'.

2. **Identify the Genotypes of the Parents**:
- Since both parents are carriers, their genotypes are:
- Parent 1: Aa (carrier)
- Parent 2: Aa (carrier)

3. **Determine the Gametes Produced by Each Parent**:
- Each parent can produce two types of gametes:
- Parent 1 can produce: A and a
- Parent 2 can produce: A and a

4. **Set Up a Punnett Square**:
- Create a Punnett square to visualize the potential offspring genotypes:
```
A a
------------
A | AA | Aa |
|-------|-------|
a | Aa | aa |
```

5. **Analyze the Offspring Genotypes**:
- The possible genotypes for the offspring from the Punnett square are:
- AA (normal)
- Aa (carrier)
- Aa (carrier)
- aa (affected by thalassemia)

6. **Count the Genotypes**:
- Out of the four possible genotypes:
- 1 is AA
- 2 are Aa
- 1 is aa (affected by thalassemia)

7. **Calculate the Probability of an Affected Child**:
- The probability of having an affected child (genotype aa) is:
- Number of affected offspring (aa) / Total number of offspring = 1/4

8. **Convert to Decimal**:
- 1/4 = 0.25

### Final Answer:
The chances of the pregnant woman resulting in an affected child are **0.25** or **25%**.

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