`"______"` is an example of X-linked recessive trait

Options

A. Phenylketonuria
B. Haemophilia
C. Cystic fibrosis
D. Sickle-cell anaemia

Answer

To determine which of the given options is an example of an X-linked recessive trait, we will analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding X-linked Recessive Traits**:
- X-linked recessive traits are caused by mutations in genes located on the X chromosome. These traits are typically expressed in males (who have one X chromosome) and may be expressed in females (who have two X chromosomes) if they inherit two copies of the mutated gene.

2. **Analyzing the Options**:
- **Option A: Phenylketonuria (PKU)**:
- Phenylketonuria is an autosomal recessive disorder. The gene responsible for PKU is located on an autosome (non-sex chromosome). Therefore, this is not an X-linked recessive trait.

- **Option B: Hemophilia**:
- Hemophilia is an X-linked recessive disorder. The gene responsible for hemophilia is located on the X chromosome. This means that males with one copy of the mutated gene will express the disorder, while females need two copies to express it. Thus, hemophilia is indeed an example of an X-linked recessive trait.

- **Option C: Cystic Fibrosis**:
- Cystic fibrosis is also an autosomal recessive disorder. The gene responsible for cystic fibrosis is located on an autosome, not on the X chromosome. Hence, it is not an X-linked recessive trait.

- **Option D: Sickle Cell Anemia**:
- Sickle cell anemia is another autosomal recessive disorder. The gene for sickle cell anemia is located on an autosome, making it not an X-linked recessive trait.

3. **Conclusion**:
- After analyzing all the options, we find that the only condition that qualifies as an X-linked recessive trait is **Hemophilia (Option B)**.

### Final Answer:
**Hemophilia** is an example of an X-linked recessive trait.

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