Which one of them is the correct statement w.r.t a gene and an allele?
a) There is one allele for a given gene which is located at a single loci in the homologous chromosome.
b) An allele is the unit of DNA responsible for the appearance and inheritance of a character.
c) An allele refers to section of DNA that controls certain traits, e.g., blood group type.
d) None of these.

Options

A. There is one allele for a given gene which is located at a single loci in the homologous chromosome.
B. An allele is the unit of DNA responsible for the appearance and inheritance of a character.
C. An allele refers to section of DNA that controls certain traits, e.g., blood group type.
D. None of these.

Explanation

To solve the question regarding the correct statement about a gene and an allele, let's analyze each option step by step.

### Step 1: Understanding Genes and Alleles
- **Gene**: A gene is a segment of DNA that contains the instructions for building proteins, which in turn influence traits. Genes are responsible for the inheritance of characteristics.
- **Allele**: An allele is a variant form of a gene. For any given gene, there can be multiple alleles, which can lead to different traits.

### Step 2: Analyzing Each Statement
1. **Statement a**: "There is one allele for a given gene which is located at a single loci in the homologous chromosome."
- **Analysis**: This statement is incorrect because a gene can have multiple alleles. Therefore, it is not true that there is only one allele for a given gene.

2. **Statement b**: "An allele is the unit of DNA responsible for the appearance and inheritance of a character."
- **Analysis**: This statement is also incorrect. It is the gene that is responsible for the appearance and inheritance of a character, not the allele.

3. **Statement c**: "An allele refers to section of DNA that controls certain traits, e.g., blood group type."
- **Analysis**: This statement is misleading. While alleles are related to traits, the definition provided is more aligned with that of a gene. A gene controls traits, and alleles are variations of that gene.

4. **Statement d**: "None of these."
- **Analysis**: Given that all the previous statements (a, b, and c) are incorrect, this statement is the correct choice.

### Conclusion
The correct answer is **d) None of these.**