In Mendel's monohybrid cross of purple-flowered and white-flowered peas, all members of the F1 generation had the phenotype because their genotype was at the flower-color locus.

All members of the F1 generation  had the Purple phenotype because their genotype was Pp at the flower locus.

Explanation:

Monohybrid cross is a cross involving one trait(for example Flower color).

When Mendel crossed true breeding  pea plants that differed in flower color ie Purple flowered pea plant(PP) is crossed with White flowered pea plant(pp),all the seeds of F1 generation were purple.

The white flower trait did not appear in the F1 generation because the purple flower trait completely masked the white flower trait . Thus ,Purple flower trait (PP) is dominant over white flower trait(pp).

So,although the genotype of F1 seeds is Pp,the phenotype is purple flower color because the allele" P" is completely dominant over the allele "p".

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