Frequency distribution of the ratio (r) between mouse and zebrafish for the relative size of two randomly picked gene families from the genomes. More specifically, for gene families i and j, r = [n_m(i)/n_m(j)]/[n_z(i)/n_z(j)] when r ≥ 1, and r = [n_z(i)/n_z(j)]/[n_m(i)/n_m(j)] when r < 1. Here, n_m(i) and n_z(i) are the numbers of putatively functional genes in gene family i of the mouse and zebrafish, respectively. Arrows show the r value for V1Rs/V2Rs and that for class II/class I ORs.