Final answer:
Phenotypes are determined by genotypes, which contain instructions for synthesizing proteins that result in observable characteristics.
Step-by-step explanation:
Individuals have different phenotypes because the underlying genotypes contain instructions for different sets of proteins. The sequence of bases within a DNA molecule represents the genetic information of the cell, with segments of DNA molecules called genes. These genes contain the instructional code necessary for synthesizing proteins, which determine an individual's phenotype. The characteristics that genes express can be physical, behavioral, or biochemical, all of which are influenced by the genes being expressed as proteins under specific conditions.