In order to know whether the phenotype is homozygous or heterozygous, a test cross is performed. As only one character of flower color is being studied, the test cross will be a monohybrid one. For this, the plant will be crossed with the plant which has the recessive trait, i.e., white flower in this case. If the offspring has white colored flowers, then the parent was heterozygous. If the offspring bears yello flowers, the the parent was homozygous for the dominant allele. In this case, a ratio of 1:1 wil be obtained in the offsprings, which means half of the population will bear white flowers and half will have yellow flowers, because recessive allele will come from both the tested corssed parents nd will obviously produce white flower plants also. On the other hand, if the parent would have been homozygous for yellow flower, all of the offsprings in the test cross yould have been yellow flower bearing plants.