To determine if a particular plant is homozygous or heterozygous you would have to test cross with a homozygous recessive. By using a homozygous recessive to test cross, the homozygous recessive parent always provides recessive gene in the gamete of the known plant. If the organism in question is heterozygous, the unknown parent will produce half of its gametes with the dominant trait and half with the recessive trait.
