How do DNA codons affect an organism's traits?
Codons code amino acids. Amino acids join to form proteins. And proteins are basis for external traits.
DNA is made of recurring nucleotides. In other words sequences of A, T, G and Cs; adenine, thymine, guanine and cytosine respectively. Three letters represent one protein. For example, ATG codes for methionine.
Consider one amino acid to be one bead. Many bead like this forms one chain of a protein. Now, as we know, proteins can be seen in almost every part of our body. Our skin, hair, nails, cell walls, enzymes, hormones....like everywhere. Thus as the codon changes, the protein expressed changes. This change can be as simple as being blond or brunette or can be life changing as being susceptible to cystic fibrosis or not.