The effect of alleles on the inheritance of a trait can be either dominant or recessive. The gene form of the dominant allele masks the effect of a recessive allele. Thus, an individual receives two alleles from his parents that affect each trait, which can be either identical or different from each other. An individual can therefore have two dominant alleles, two recessive alleles, or one dominant and one recessive allele for a gene.

The trait produced by the dominant allele always appears in the individual. This is true whether the individual is heterozygous and homozygous. The recessive form of the trait, in turn, is manifested only if it is homozygous.

For example, the composition of a human's ear wax can be of two types: dry or moist. The dominant allele of the trait causes an individual to have moist ear wax, whereas the recessive allele causes the individual to have dry ear wax. If an individual inherits a recessive allele (a) from both parents, their ear wax is dry because their inheritance is (aa). If, on the other hand, one or both parents pass on the dominant allele (A), the individual has moist ear wax because their inheritance is (Aa) or (AA).

The composition of ear wax

Individual	Genotyoe	Phenotype
Homozygous (dominant allele)	AA (i.e., an individual has inherited an A allele from both parents)	moist ear wax
Heterozygous	Aa	moist ear wax
Homozygous (recessive allele)	aa	dry ear wax