Final answer:
If a heterozygous organism exhibits a mutant phenotype, the mutant allele is considered dominant. This allele overrides the presence of a wild-type allele, causing the mutant phenotype to be expressed in heterozygous individuals.
Step-by-step explanation:
If a heterozygous organism shows a mutant phenotype, the mutant allele is considered to be dominant. A dominant allele expresses its phenotype even when only one copy is present alongside a normal, or wild-type, allele. The phenomenon where a single copy of a mutant allele affects the organism's traits even in the presence of a functional allele is a hallmark of dominant inheritance.
In contrast, a recessive allele only manifests its phenotype when an organism is homozygous for that trait—meaning the organism carries two copies of that recessive allele. Therefore, for a gene with both a dominant and a recessive allele, the heterozygous (one copy of each allele) and homozygous dominant (two copies of the dominant allele) organisms will appear the same, displaying the dominant trait.
As an example, in Drosophila flies, the Antennapedia mutation is dominant over the wild type because heterozygous flies with one mutant allele will develop legs on their head where antennae should be. This is due to the mutant allele altering the expression or function of the gene product, thus interfering with the normal, wild-type phenotype even in the presence of a wild-type allele.