The study found that most mutations in yeast genes do not have a significant impact on fitness. The researchers argue that previous results suggesting otherwise were due to a lack of proper controls for background genetic effects. They showed that both synonymous and nonsynonymous mutations have minimal effects on fitness. The wide range of fitness effects observed for certain mutations may be due to other biological factors not accounted for in the study. The study emphasizes the importance of proper experimental design to accurately assess the effects of mutations on fitness.