Determination of four groups of ohnologs based on (i) correlation between their expression patterns (HC: high correlation, p > 0.05; NC: no correlation, p < 0.05, Pearson’s correlation test), and (ii) their relative expression levels (SE: similar expression levels, p > 0.05; DE: different expression levels, p < 0.05, Student’s t-test). The percentage of genes present in each class (HCSE, HCDE, NCSE, and NCDE) is shown for zebrafish, medaka, and for genes belonging in the same class for the two species.