With regard to the gene for flower color, red is dominant to white. This gene demonstrates complete dominance.

Question

If you have a red flowered plant, explain how would you determine its genotype?

Answer 1

You could perform a test cross by mating the red-flowered plant with a white-flowered plant.

If all the offspring have red flowers, the parent plant is homozygous dominant.  If half of the offspring turn out red, and half turn out white, then the parent plant is heterozygous.