The two alleles of a gene occupy corresponding positions on homologous chromosomes and control the same character (for example, the height of a plant) but produce different effects (short or tall).
A dominant allele hides a recessive allele's expression. An uppercase letter is used to signify it.
A recessive allele exerts its effect only in the homozygous state. It is represented by a lower case letter.
Some examples of alleles:
The dominant purple and recessive white alleles are responsible for flower color in pea plants.