When two different alleles occur together in a heterozygote, their phenotypic relationship can sometimes be described by complete dominance.

Suppose alleles $A$ and $a$ affect one phenotype. If the heterozygote $Aa$ has the same phenotype as $AA$, then $A$ is dominant to $a$ for that phenotype, and $a$ is recessive to $A$.

For example:

AA → purple flowers
Aa → purple flowers
aa → white flowers

Here the phenotype associated with $A$ appears in both $AA$ and $Aa$ individuals.

Dominant does not mean common, stronger or better

Dominance describes a relationship between alleles within a heterozygote. It does not imply that the dominant allele

• is more frequent in a population;
• improves survival or reproductive success;
• has a physically larger DNA sequence;
• arose earlier;
• or will eventually replace a recessive allele.

A rare allele can be dominant, and a common allele can be recessive.

Recessive phenotypes reveal genotype more directly

Under complete dominance, an individual showing the recessive phenotype must normally be homozygous for the recessive allele:

$$aa.$$

An individual showing the dominant phenotype can have either

$$AA\quad\text{or}\quad Aa.$$

The phenotype alone therefore does not always determine genotype.

Dominance is phenotype-specific

One allele can be dominant for one measured characteristic but not show the same relationship for every possible molecular or organismal measurement. The classification depends on the phenotype being compared.

The key rule is: under complete dominance, the heterozygote has the same measured phenotype as one homozygote, and that homozygote's allele is called dominant for that phenotype.