● What's a Codon: Codon is a sequence of three DNA or RNA characters. Each codon codes the specific amino acid. This amino acids are construction elements of proteins. There are two special amino acids for protein start and protein end.

● 1st & 2nd versus 3rd Codon position: each codon does code some amino acid. There are 4^3=64 possible combinations, therefore 64 possible codons, but only 20 amino acids. Many amino acids can be coded with slightly different codons. Mainly the third position differs. Therefore the mutations on the third position can cause no effect on produced amino acid, that would cause no effect on organism. Also, the mutations on the 3rd Codon position are not so harmful as the mutations on 1st und 2nd position, and therefore more likely to stay in the population.

● Synonymous versus non-synonymous substitutions: synonymous substitution - substitution on some codon position (at most the 3rd), that causes the producing of same amino acid. Non-synonymous substitution - substitution that causes the production of another amino acid.