In genetics, dominance pertains to the property of a gene (or allele) in relation to other genes or alleles. A gene or allele shows dominance when it suppresses the expression, or dominates the effects, of the recessive gene (or allele). There are many forms of dominance: complete dominance, incomplete dominance, and codominance.
Complete dominance is a form of dominance in heterozygous condition wherein the allele that is regarded as dominant completely masks the effect of the allele that is recessive. For instance, an individual carrying two alleles that are both dominant (e.g. AA), the trait that they represent will be expressed. But if the individual carries two alleles in a manner that one is dominant and the other one is recessive, (e.g. Aa), the dominant allele will be expressed while the recessive allele will be suppressed. Hence, the heterozygote (Aa) will have the same phenotype as that of the dominant homozygote (AA). This condition is called complete dominance.
When the dominance is not complete, it is referred to as incomplete dominance. In this form of dominance, the dominant allele is only partially expressed. The result is a heterozygote (Aa) with an intermediate phenotype. In another form of dominance, i.e. codominance, the alleles of a gene pair in a heterozygote are fully expressed. This results in offspring with a phenotype that is neither dominant nor recessive.