What is the primary factor that determines an organism's phenotype?

The primary factor that determines an organism's phenotype is the genotype. The genotype refers to the specific genetic makeup of an organism, encompassing all of its alleles, which ultimately dictate how traits are expressed. Essentially, the genotype serves as the blueprint for the organism, guiding the development of physical characteristics (phenotype) such as color, size, and shape.

Although environmental conditions can influence the expression of certain traits, meaning that the phenotype can vary based on different environments, the underlying genetic code set forth by the genotype is fundamentally what dictates those traits. Genetic mutations and allelic variations can indeed contribute to differences in phenotype, but they are components of the genotype itself. Thus, the genotype is the most comprehensive factor in determining an organism's observable characteristics.