How might similarities and differences in genetic codes, or the proteins built as a result of these codes, be used to determine how closely related different species are?

There are 4 nitrogen used in the construction of codon. Each codon is composed of 3 bases. Hence there are 4^3 or 64 codons available. In this case, we combination is distinguishable from one to another and represents another thread of amino acid. The building blocks of DNA is nucleotide is based on these codons. Hence the closer the codon make-up, the closer the genetics are.