Figure 2.  Nucleotide/amino acid difference versus years difference of the influenza A strains from the 1968 and the 1918 pandemic lineage to the corresponding parent. Left two panels are for the 1968 pandemic lineage with parent A/Hong Kong/1/1968 (H3N2). The lineage contains human H3N2 (“.” mark), North American swine H1N2 (“x” mark), and 2009 human H1N1 (“x” mark). Right two panels are for the 1918 pandemic lineage with parent A/Brevig Mission/1/1918 (H1N1) containing human H1N1, except 2009 human H1N1. Top two panels are nucleotide difference, bottom two are amino acid difference. In each panel strains in which PB1-F2 is 90 amino acids long are marked with “+”.