For an individual having a genotype formed of two different alleles that condition different varieties of the same phenotypical trait, upon what will the phenotypical feature actually manifested depend?
If an individual presents the gene having different alleles (common situation), for instance, A and A', three kind of genotypes may be formed: AA, A'A' and AA'. The question demotes to an individual bearing a genotype made of two dissimilar alleles, so it is the AA' genotype (the heterozygous individual).
This AA' individual may possibly manifest the phenotype conditioned by the allele A or the phenotype conditioned by the allele A' or still a mixed phenotype of those two forms. If the allele A is the dominant over the allele A' the form conditioned by A will manifest. If A' is dominant allele, the form determined by A' will manifest. This phenomenon is called as dominance and occurs because the recessive (nondominant) allele manifests only when present in double in the genotype (in homozygosity), while the dominant allele manifests even when in heterozygosity. If not any of the alleles dominate a mixture of the two varieties conditioned by both alleles appears or instead a third form may come out.