A single recessive allele which can express its effect should occur on

Options

A. Any autosome
B. Any chromosomes
C. X-chromosome of female
D. X-chromosome of male

Answer

To solve the question regarding where a single recessive allele can express its effect, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Recessive Alleles**:
- A recessive allele is one that does not manifest its trait in the presence of a dominant allele. It can only express its effect when present in a homozygous state (i.e., two copies of the recessive allele).

2. **Identifying Chromosome Types**:
- Chromosomes can be categorized into autosomes (non-sex chromosomes) and sex chromosomes (X and Y chromosomes). In humans, there are 22 pairs of autosomes and 1 pair of sex chromosomes.

3. **Analyzing the Options**:
- **Any Autosome**: A recessive trait on an autosome requires two copies (one from each parent) to express. Therefore, this option is incorrect if we are considering a single recessive allele.
- **Any Chromosome**: This is too broad and does not specify the conditions needed for expression, making it incorrect as well.
- **X Chromosome of Female**: Females have two X chromosomes. For a recessive trait to express, they would need two copies of the recessive allele on both X chromosomes, so this option is also incorrect.
- **X Chromosome of Male**: Males have one X and one Y chromosome (hemizygous for X). If a recessive allele is present on the X chromosome, it will express its effect because there is no dominant allele on the Y chromosome to mask it.

4. **Conclusion**:
- The correct answer is that a single recessive allele can express its effect on the **X chromosome of a male**.

### Final Answer:
A single recessive allele which can express its effect should occur on the **X chromosome of male**.

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