The color purple (P) is dominant in a plant. If you have a purple plant with an unknown genotype and cross it with a white plant (pp) and get offspring which are 100% purple, what is the genotype of the unknown plant?

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Before we walk through this explanation together, let's review some important terms to know:
Allele: An allele is one of the possible forms of a gene. Most genes have two alleles. An allele can either be a dominant allele (represented by a capital letter in a Punnett square) or a recessive allele (represented by a lowercase letter in a Punnett Square).
Homozygous: Having two of the same alleles (both dominant or both recessive) of a particular gene. If both alleles are dominant, then the dominant trait is expressed. If both alleles are recessive, then the recessive trait is expressed.
Heterozygous: Having two different alleles (one dominant and one recessive) of a particular gene. In this case, the dominant trait would be expressed.
Genotype: The genetic makeup of an organism (e.g. bb, Bb, or BB)
Phenotype: How the genetic makeup of an organism expresses itself (e.g. brown hair, blonde hair, red hair).
The best way to determine the genotypes of parents or offspring is to draw Punnett Squares to represent each possibility.
In order for all the plants to have purple flowers, there can be no recessive alleles in the original (parent) purple plant. Since the purple allele is dominant, we know that the white plant must have two recessive alleles, otherwise it would be purple. The cross between these two is represented in the Punnett Square below:
 11ec0fda_9973_9a61_a67f_7d5a01836068_NR0004_00 If the parent purple plant's genotype was Pp, however, the cross between the two would look like this:
 11ec0fda_9973_9a62_a67f_f77e032202d6_NR0004_00 If the purple plant was Pp, then 50% of the offspring would have white flowers because their genotype would be pp. Since all of the offspring have purple flowers, we can conclude that the original (parent) purple plant had to be PP.

The Color Purple (P) Is Dominant in a Plant