A unified model coping with both physical and optical dot gains on print color tone reproduction is proposed. The physical dot gain, is approximated by a quadratic function of nominal dot percentages. The function, for each color, is characterized by a single parameter depending on printing technologies as well as ink-paper interactions, and has a symmetric form around where the nominal dot percentage is 50%. The parameter can be derived from the best fit for the model to measured data, such as spectral reflectance values or CIEXYZ tristimulus values. Applications to a color laser printer (on office copy paper) reproduces the experimental dot gain curves fairly well. Dependence on physical dot percentage, a summation of the nominal dot percentage and the corresponding physical dot gain, results in the optical dot gain and in turn the overall dot gain asymmetric forms, plotted against the nominal dot percentages. Furthermore, theoretical analysis reveals fundamental differences between physical and optical dot gains. Therefore, effects of optical dot gain can not be accurately represented by any physical extension in printing practices.