How can you determine the genotype of a parent that has a dominant trait?

To determine the genotype of a parent exhibiting a dominant trait, mating that parent with a homozygous recessive parent is an effective approach. This method is known as a test cross. When the dominant parent (whose genotype is unknown) is crossed with a homozygous recessive parent, the phenotypes of the offspring reveal crucial information about the dominant parent's genotype.

If all offspring display the dominant trait, it indicates that the dominant parent must possess at least one dominant allele, meaning it could be either homozygous dominant or heterozygous. However, if any offspring exhibit the recessive trait, it confirms that the dominant parent is heterozygous, carrying one of each allele. Thus, the test cross provides a clear insight into the unknown genotype based on the resulting phenotypes of the offspring, making it the most reliable method among the options provided.

The other methods, while useful in different contexts, do not directly confirm the genotype as effectively as a test cross.