Testcross.
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   An  organism  exhibiting  a  dominant  trait  (purple flowers). can be
   either homozygous dominant or heterozygous.
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   To  determine its genotype, the individual with the dominant phenotype
   is  crossed  with  a  recessive  phenotype  (white flowers), since the
   latter's genotype has to be homozygous recessive.
   By  observing  the  phenotypes  of  the  offspring,  we can deduce the
   genotype of the purple-flowered parent.