What is the pattern of inheritance for a sex-linked allele ?

Options

A. Every affected person has an affected parent
B. Unaffected parents cam produce children who are affected
C. Unaffected mothers have affected sons and daughters who are carriers
D. None of these

Answer

**Step-by-Step Solution:**

1. **Understanding Sex-Linked Alleles**:
- Sex-linked alleles are genes located on the sex chromosomes (X and Y). In humans, the X chromosome carries many genes, while the Y chromosome carries fewer. Most sex-linked traits are associated with genes on the X chromosome.

2. **Analyzing the Options**:
- **Option A**: "Every affected person has an affected parent."
- This statement is not necessarily true for sex-linked traits. An affected male can have an unaffected mother, as males inherit their X chromosome from their mother.

- **Option B**: "An unaffected parent can produce children who are affected."
- This is also incorrect in the context of sex-linked inheritance. If a parent is unaffected, they cannot pass on the affected allele unless they are a carrier (which is not the case for males).

- **Option C**: "Unaffected mothers have affected sons and daughters who are carriers."
- This is a correct statement. An unaffected mother (XX) can have affected sons (XY) if the father is affected (X^aY), and the daughters can be carriers (X^aX) if they inherit the affected X from their father.

- **Option D**: "None of these."
- This option is incorrect since option C is a valid statement.

3. **Conclusion**:
- The correct answer is **Option C**: "Unaffected mothers have affected sons and daughters who are carriers."