A man and a woman, who do not show any apparent signs of a certain inherited disease, have seven children (2 daughters and 5 sons). Three of the sons suffer from the given disease but none of the daughters are affected. Which of the following mode of inheritance do you suggest for this disease ?

Options

A. Autosomal dominant
B. Sex-linked dominant
C. Sex-limited recessive
D. Sex-linked recessive

Explanation

To determine the mode of inheritance for the disease in question, we can analyze the information provided step by step.

### Step-by-Step Solution:

1. **Understanding the Family Structure**:
- We have a man and a woman who do not show any signs of the inherited disease.
- They have seven children: 2 daughters and 5 sons.
- Among the sons, 3 are affected by the disease, while none of the daughters are affected.

2. **Constructing a Pedigree**:
- Draw a simple pedigree chart to visualize the family.
- Represent the man as a square (♂) and the woman as a circle (♀).
- Indicate the affected sons with a filled square and unaffected individuals with an empty shape.
- The pedigree will show the parents with two daughters (both empty circles) and five sons (three filled squares and two empty squares).

3. **Analyzing the Inheritance Pattern**:
- Since the parents are unaffected, they cannot be exhibiting an autosomal dominant trait, as that would require at least one parent to show symptoms.
- For an autosomal dominant trait, affected individuals typically have at least one affected parent.

4. **Considering Sex-Linked Inheritance**:
- The disease could be sex-linked, meaning it is carried on the X chromosome.
- In this scenario, if the trait were sex-linked dominant, the mother would need to be affected or a carrier, which would lead to affected daughters as well, contradicting the information given.

5. **Determining the Mode of Inheritance**:
- The most plausible explanation is that the disease is sex-linked recessive.
- In this case, the mother could be a carrier (X^dX) and the father would have the normal Y chromosome (XY).
- The affected sons inherit the X chromosome with the disease allele (X^d) from their mother, while the daughters inherit the normal X chromosome from both parents.

6. **Conclusion**:
- Therefore, the mode of inheritance for the disease is **sex-linked recessive**.
- In sex-linked recessive inheritance, males are more likely to be affected, while females can be carriers without showing symptoms.