We also tested, individually, if duplication genes, deletion genes, and mixed genes tended to be under relaxed selective constraint compared to neutral genes. Both duplication and mixed genes were shown to have significantly higher dN/dS ratios than neutral genes, while dN/dS ratios of deletion genes did not differ significantly from those of neutral genes. The reduction in selective constraint observed in duplication and mixed genes follows Ohno's hypothesis that in a gene duplication event, one or both duplicates should experience relaxed selective constraint resulting in elevated rates of sequence evolution.