The purpose of this study is to examine high school students' understanding of the Law of Segregation by introducing the concept of Incomplete Dominant Trait in the introduction of genetics in science education. This study revealed that by introducing Incomplete Dominant Trait, students attained a better understanding of the Law of Segregation. In addition, the majority of students took the F1 trait as the basis of the assumed gene. This study was conducted with a control group (38 high school 2nd graders) and an experimental group (24 high school 1st graders). In order to examine students' understanding, both groups were given a paper-pencil test which related to the Law of Segregation. This study concluded that learning about the Law of Segregation by introducing the concept of Incomplete Dominant Trait had a positive effect on students' understanding.