Changes in the DNA are reflected in the protein product. There is a correlation between what happens to the genetic sequence and the resulting protein sequence.
DNA has four "letters" that must specify the 20 different amino acids that make up proteins. Combinatorially, using three DNA letters for one amino acid makes the most sense.
A human is a complicated organism, and it is considered unethical to do many kinds of experiments on human subjects. For these reasons, biologists often use simpler “model” organisms that are easy to keep and manipulate in the laboratory.