Haemophilia is a condition in which blood fails to clot or clots only very slowly. Studies of this human sex-linked trait show that

Options

A. every X chromosome carries the dominant allele.
B. a Y chromosome never carries the dominant allele.
C. both X and Y chromosomes can bear the recessive allele.
D. neither X nor Y chromosomes can bear the recessive allele.

Answer

### Step-by-Step Solution:

1. **Understanding Hemophilia**: Hemophilia is a genetic disorder characterized by the inability of blood to clot properly. This condition is primarily linked to the X chromosome.

2. **Genetic Basis**: The gene responsible for hemophilia is located on the X chromosome. There are two types of alleles for this gene:
- Normal allele (represented as X)
- Defective allele (represented as XH)

3. **Genotypes in Females**: Females have two X chromosomes (XX). The possible genotypes for females regarding hemophilia are:
- Normal (XX)
- Carrier (XHX)
- Hemophilic (XHXH)

4. **Genotypes in Males**: Males have one X and one Y chromosome (XY). The possible genotypes for males regarding hemophilia are:
- Normal (XY)
- Hemophilic (XHY)

5. **Inheritance Patterns**: Since males have only one X chromosome, if they inherit the defective allele (XH), they will express hemophilia. Females can be carriers or affected depending on whether they inherit one or two defective alleles.

6. **Evaluating Statements**: Based on the understanding of hemophilia and its genetic basis, we can evaluate the statements:
- **Statement 1**: Every X chromosome carries the dominant allele. (Incorrect)
- **Statement 2**: A Y chromosome never carries the dominant allele. (Correct)
- **Statement 3**: Both X and Y chromosomes can bear the recessive allele. (Incorrect)
- **Statement 4**: Neither X nor Y chromosome can bear the recessive allele. (Incorrect)

7. **Conclusion**: The correct statement based on the observations is that the Y chromosome never carries the dominant allele.