Fig. 2
From: The optimality of the standard genetic code assessed by an eight-objective evolutionary algorithm
The scheme of the crossover operator for the US model. a Two offspring O1 and O2, which are identical to their corresponding parents P1 and P2, are created. The same amino acid in P1 and P2, e.g. a1 is randomly selected. If this amino acid is encoded by the same codons (the orange arrow), no exchange of these codons is performed between O1 and O2 (the black arrow). b If this amino acid is encoded by different codons in P1 and P2 (the orange arrow), these codons are exchanged mutually within O1 and O2 (the black arrows). c If there are no codons for the amino acid a1 in one parent to exchange, e.g. P1 but the second parent, e.g. P2 has still a codon for this amino acid (the orange arrow), this codon is moved from other amino acid, e.g. a3, and assigned to a1 in one offspring, e.g. O1 (the black arrow). In the second offspring O2, this codon is moved from the amino acid a1 and assigned to the other amino acid a3 (the black arrow). d Codons that are the only ones for the given amino acid in parents (the orange arrow) are not moved in the offspring (the black arrow with the red x)