A plant whose genotype is represented with the alleles TT has the phenotype of a tall plant. Since T represents a tall plant and it is the only type of allele present in the plant's genotype, the plant phenotypically manifests as a tall plant.
An allele is an alternative form of a gene for a particular inheritable characteristic. A plant's height characteristic can be represented by the alleles T for tall and t for short. The genotype of a plant refers to the allele combination carried by the plant, while phenotype refers to the outward physical appearance of the plant. A plant such as the one whose genotype is represented by TT is said to be homozygous. This is because both of its alleles are identical. A plant whose genotype is represented by alleles that are not identical, such as one with the combination Tt, is said to be heterozygous.
These concepts of genetics were developed in the 1860s by Gregor Mendel. He made his discoveries by performing methodical experiments on thousands of garden pea plants. He opted for pea plants because they were easy to grow, reproduced quickly and had easily distinguishable characteristics. Some of the characteristics which Mendel observed included stem height, flower color and position, and both seed and pod shape and color.