Final answer:
In the context of Mendelian inheritance, the mystery parent crossed with a purple-stemmed parent that produces all purple-stemmed offspring must have at least one dominant allele for the trait. Therefore, the mystery parent's genotype must be either heterozygous (Pp) or homozygous dominant (PP), but cannot be homozygous recessive (pp) for stem color.
Step-by-step explanation:
From the information given, we know that when a parent with purple stems is crossed with a mystery parent, all the offspring have purple stems. This scenario suggests a typical Mendelian inheritance pattern of dominant and recessive alleles. In such a cross, purple stem coloration is likely due to a dominant allele (we can represent it as 'P').
Given that 100% of the offspring display the purple phenotype, the mystery parent must also possess at least one dominant allele 'P' to contribute to this phenotype. This is because the presence of the dominant allele in both parents would result in all purple offspring, as any offspring receiving a 'P' allele from either parent would be purple. Therefore, the mystery parent’s genotype must be either heterozygous (Pp) or homozygous dominant (PP) for the stem color trait.
The fact that all offspring are purple rules out the possibility that the mystery parent could be homozygous recessive (pp), as that would result in some offspring displaying the recessive phenotype if the known purple parent were heterozygous (Pp).