Alleles are

Options

A. Different phenotype
B. True breeding homozygotes
C. Different molecular forms of a gene
D. Heterozygotes

Answer

**Step-by-Step Solution:**

1. **Understanding Alleles**: Alleles are defined as different forms of a gene. Each gene can have multiple variations, which are referred to as alleles. For example, a gene that determines flower color may have a purple allele and a white allele.

2. **Location of Alleles**: Alleles are located at specific positions (loci) on homologous chromosomes. Homologous chromosomes are pairs of chromosomes that have the same structure and gene sequence, but may contain different alleles.

3. **Examples of Alleles**: If we consider a gene represented by "A", it can have different alleles such as "A" (dominant) and "a" (recessive). These alleles can exist at the same locus on homologous chromosomes.

4. **Analyzing the Options**:
- **Different Phenotypes**: This option is incorrect because phenotypes refer to the observable traits or characteristics of an organism, not the alleles themselves.
- **True Breeding Homozygotes**: This option is also incorrect. True breeding homozygotes refer to individuals that have two identical alleles for a trait (e.g., AA or aa), but this does not define what alleles are.
- **Different Molecular Forms of a Gene**: This option is correct. It accurately describes alleles as the different molecular forms of a gene.
- **Heterozygotes**: This option is incorrect. Heterozygotes refer to individuals that have two different alleles for a trait (e.g., Aa), but this does not define alleles themselves.

5. **Conclusion**: Based on the analysis of the options, the correct answer is that alleles are "different molecular forms of a gene."

**Final Answer**: Alleles are different molecular forms of a gene.

---