Ans: The order of codons
Explanation: The DNA has a particular sequence of the dNTPs. At the time of gene expression, the DNA is transcribed into the mRNA. The sequence of three nucleotides on the mRNA makes the codons, that codes for a particular amino acid. These codons form the genetic code. Change in the order of codons results in the different order of the amino acids and thus different proteins. Proteins are responsible for particular phenotypes (external appearance) hence change in proteins creates differences between the organisms.