Alleles are

Options

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

Answer

**Step-by-Step Solution:**

1. **Definition of Alleles**: Alleles are defined as different molecular forms of a gene. Each gene can have multiple forms, which are referred to as alleles.

2. **Types of Alleles**: Typically, there are two alleles for a gene, but in some cases, there can be more than two forms, known as multiple alleles.

3. **Understanding Heterozygotes**: Heterozygotes are individuals that have two different alleles for a particular gene—one dominant allele and one recessive allele.

4. **Genotypes and Phenotypes**: The genotype of an organism (the genetic makeup) determines its phenotype (the observable traits). Different combinations of alleles lead to different phenotypes.

5. **True Breeding Homozygotes**: True breeding homozygotes are individuals that have two identical alleles for a gene, either both dominant or both recessive. They do not exhibit heterozygous conditions.

6. **Conclusion**: Based on the definitions and explanations provided, the correct answer to the question "Alleles are" is that they are different molecular forms of a gene.