A dominant allele is an allele that is expressed in the phenotype of an individual, even if the individual is heterozygous for the gene. This means that if an individual has one copy of the dominant allele and one copy of the recessive allele, the dominant allele will be expressed. Recessive alleles, on the other hand, are only expressed in the phenotype of an individual if the individual is homozygous for the gene. This means that if an individual has two copies of the recessive allele, the recessive allele will be expressed.

The difference between dominant and recessive alleles is due to the way that they interact with each other. Dominant alleles produce a protein that is functional, while recessive alleles produce a protein that is non-functional. When an individual is heterozygous for a gene, the functional protein produced by the dominant allele will be enough to produce the normal phenotype. However, when an individual is homozygous for a recessive allele, the non-functional protein produced by the recessive allele will not be able to produce the normal phenotype. This will result in the expression of the recessive phenotype.

Here is a table summarizing the key differences between dominant and recessive alleles:

| Characteristic | Dominant Allele | Recessive Allele |

|---|---|---|

| Expression in heterozygotes | Expressed | Not expressed |

| Expression in homozygotes | Expressed | Expressed |

| Protein function | Functional | Non-functional |

| Phenotype in heterozygotes | Normal | Normal |

| Phenotype in homozygotes | Normal | Recessive |