Final answer:
The term for the complete genetic makeup of an organism is 'genotype,' consisting of two alleles per trait, which can be homozygous or heterozygous. Observable traits are known as the phenotype.
Step-by-step explanation:
The term that refers to the complete genetic makeup of an organism, with each trait represented by two letters (alleles), is genotype. An organism's genotype consists of the alleles it has inherited from its parents, with one allele on each chromosome of a homologous pair. For example, the genotype could be represented by letter combinations such as TT, Tt, or tt. Organisms with two identical alleles for a trait (TT or tt) are homozygous, while those with different alleles for a trait (Tt) are heterozygous.
The observable traits expressed by an organism, such as hair color or pod color in peas, are referred to as its phenotype. While the phenotype is influenced by the genotype, the two are not identical. This concept can be seen in Mendel's hybridization experiments, where true-breeding plants with different pod colors were cross-fertilized, leading to F1 offspring with a uniform phenotype but genetically differing from the parent (since some F2 offspring showed the recessive trait).