Fully dominant genes will be expressed phenotypically whether they are in a homozygous or heterozygous condition. However, only the homozygotes will breed true and sometimes it is desirable to determine which individuals in a population are true breeders and which ones are the heterozygotes. To determine which individuals are true breeders, a test cross is run. It involves mating a genotypically unknown individual with one whose genotype is known. In a situation where dominance is complete, what type of individual will carry a genotype that is known beyond any doubt? Think about that for a few moments before clicking the answer button. Then flip to the next card.