Final answer:
A pea plant described as "tall" exemplifies its phenotype, which is the observable trait resulting from its genotype. Both homozygous dominant (TT) and heterozygous (Tt) genotypes lead to a tall phenotype in pea plants due to the dominant allele for tallness.
Step-by-step explanation:
When a pea plant is described as "tall", that description is referring to its phenotype, which is the set of observable characteristics of an organism. The genotype, on the other hand, is the genetic makeup that determines these traits. In the case of Mendel's pea plants, the genotype TT (homozygous dominant) or Tt (heterozygous) would both result in a tall phenotype because the allele for tallness (T) is dominant over the allele for shortness (t).
Therefore, whether the plant has two dominant alleles (TT) or one dominant and one recessive allele (Tt), the phenotype will be tall. On the contrary, a plant must have two recessive alleles (tt) to exhibit the short phenotype, demonstrating how different genotypes can produce the same phenotype. This distinction exemplifies the relationship between phenotypes and genotypes in the expression of physical characteristics.