Traits, which we define as visible aspects of an organism, are coded for by an organisms DNA and are expressed through the proteins that the DNA codes for. The expression of "traits" can be traced back specifically to the order of nucleic bases in the DNA, and the proteins those bases code for. DNA codes for RNA which then codes for specific amino acids.
(DNA codes for mRNA in a process called transcription. Then, that mRNA is translated at ribosomes. The anticodons that bind to the mRNA determine which proteins are made. Ultimately, those proteins express traits based on the function of the protein.)