From Table 18.2, are mutations in the first or second base of a codon more or less serious than mutations at the third base?

Table 18.2The Genetic Code; Translation of the Codons into Amino Acids Third base (3' end) First base (5' end) Middle base Phe Ser Tyr Cys Phe Ser Tyr Leu Ser Stop Stop Leu Leu Ser Stop Trp Leu Leu Pro His Arg Leu Gln Gin His Arg Ile Thr Asn Ser lle Thr Asn Ser Val Met (start) Thr Lys Lys Val Val Ala Asp Gl Val Ala Glu Gly Asp Gly Glu Gly