Final answer:
The phenotype ratio in the F2 generation demonstrates that the CDK-Mut allele is recessive if it shows a 3:1 dominant to recessive ratio, as only the homozygous recessive plants will exhibit the mutant phenotype.
Step-by-step explanation:
To explain how the phenotype ratio of the F2 generation demonstrates that the cyclin-dependent kinase-mutant (CDK-Mut) allele is recessive, we need to consider Mendelian genetics. First, plants that are homozygous for the CDK-WT allele are crossed with plants homozygous for the CDK-Mut allele, producing F1 offspring that are all heterozygous, assuming the WT allele is dominant. When these heterozygous plants are crossed, the resulting F2 generation is expected to show a 3:1 dominant to recessive phenotypic ratio, as Mendel observed in his experiments with pea plants.
In such a cross, the expected genotypic ratio would be 1 homozygous dominant (CDK-WT/CDK-WT) : 2 heterozygous (CDK-WT/CDK-Mut) : 1 homozygous recessive (CDK-Mut/CDK-Mut). However, because the CDK-Mut allele is recessive, both homozygous dominant and heterozygous plants will show the wild-type phenotype, while only the homozygous recessive plants will show the mutant phenotype. This results in a phenotypic ratio of 3 wild-type to 1 mutant, which indicates that the CDK-Mut allele does not affect the phenotype unless it is in a homozygous state. If CDK-Mut were dominant, we would not see a 3:1 ratio but instead a 1:2:1 ratio of wild-type to mutant phenotypes.