Final answer:
The observable expression of a trait, known as the phenotype, is determined by an organism's genotype and its interaction with the environment. The genotype consists of the alleles that an organism inherits, which may result in visible traits or may be non-expressed but still contribute to the organism's phenotype.
Step-by-step explanation:
The observable expression of a trait in an organism is determined by its phenotype. The phenotype is the set of observable characteristics or traits, which includes factors such as color and structural morphology, and is the result of the interaction between the organism's genotype and its environment. The genotype, on the other hand, refers to the genetic makeup of an organism; it is the combination of alleles that the organism possesses.
For example, in Mendel's hybridization experiments, when true-breeding plants with different colored pods were cross-fertilized, the F1 hybrid offspring exhibited only one of the parental traits, which was their phenotype. However, the underlying genetic makeup (genotype) of the F1 hybrids contained alleles for both traits, as evidenced by the reappearance of the recessive trait in some of the F2 offspring.