Explain why an organism with a homozygous dominant genotype has the same phenotype as an organism with a heterozygous genotype

In phenotype (visible physical characteristics), only dominant/recessive will show. If an organism is homozygous dominant (e. g. AA), then the dominant allele will result in a dominant phenotype. If an organism if heterozygous dominant (e. g. Aa), then even though it has a recessive gene, the dominant allele will overpower the recessive one, resulting in only the dominant trait showing in the phenotype.