Same gene loci on two homologous chromosomes produce different phenotypes. They bear

Options

A. Homozygous alleles
B. Heterozygous alleles
C. Two different genes
D. Pleiotropic genes.

Answer

To solve the question, "Same gene loci on two homologous chromosomes produce different phenotypes. They bear," we need to analyze the options provided and understand the concepts involved.

### Step-by-Step Solution:

1. **Understanding Gene Loci**:
- Gene loci refer to the specific locations of genes on homologous chromosomes. Each gene can have different forms, known as alleles.

2. **Identifying Alleles**:
- Alleles are variants of a gene that can produce different traits or phenotypes. For example, a gene for flower color might have a purple allele and a white allele.

3. **Homologous Chromosomes**:
- Homologous chromosomes are pairs of chromosomes that have the same structure and gene sequence but may carry different alleles.

4. **Analyzing the Phenotypes**:
- If the same gene loci on two homologous chromosomes produce different phenotypes, it indicates that the alleles present at these loci are different.

5. **Evaluating the Options**:
- **Homozygous alleles**: This means both alleles at a locus are the same (e.g., AA or aa). This would not produce different phenotypes.
- **Heterozygous alleles**: This means the alleles at a locus are different (e.g., Aa). This can produce different phenotypes.
- **Two different genes**: This would imply that the loci are not the same, which contradicts the question.
- **Pleiotropic genes**: These are genes that can affect multiple traits, but the question specifies different phenotypes at the same loci, which is not the primary definition of pleiotropy.

6. **Conclusion**:
- Since the question states that the same gene loci on homologous chromosomes produce different phenotypes, the correct answer is **heterozygous alleles**. This is because heterozygous alleles can lead to different expressions of a trait, resulting in different phenotypes.

### Final Answer:
The correct option is **heterozygous alleles**.