Final answer:
Geneticists refer to the alleles Tt as a genotype, which is the genetic makeup of an organism. The trait 'small' is considered a phenotype, which is the observable characteristic resulting from the genotype's interaction with the environment.
Step-by-step explanation:
When discussing genetics and heredity, it's important to understand the distinction between genotype and phenotype. A genotype is the genetic makeup of an organism, the specific combination of alleles an individual possesses. For instance, the alleles Tt represent a heterozygous genotype, where "T" signifies the dominant allele and "t" the recessive. The word "heterozygous" indicates that the two alleles are different – one dominant and one recessive. Conversely, a phenotype is the observable characteristic or trait of an organism, such as height, flower color, or in this case, the trait 'small'. It's what is manifested physically and can be seen as a result of the interaction between the genotype and the environment. The phenotype 'small' in this question is likely the result of a recessive allele's expression when two such alleles are paired (tt), given that 'T' is a dominant allele.
In summary, the pair of alleles represented as Tt constitutes the genotype of an individual, while the expression of a trait, such as 'small', is described as the phenotype.