Genetics as a core concept of life science is essential for understanding biology. Examining genetics understanding among biology majors is becoming important since they must necessarily achieve some level of genetics understanding to advance their career. This study compares Indonesian biology majors' genetics understanding with previously published data from students in the United States (US). This study also identifies the effect of academic year and program on genetics understanding by administering the Genetics Concept Assessment (GCA) to 377 biology majors in Indonesia. IRT-Rasch modeling was performed for instrument validation, followed by one-way ANOVA and independent sample t-test for the analysis. The results showed that Indonesian biology majors' genetics understanding was significantly affected by academic year but was not affected by the academic program. Indonesian biology majors had a slightly similar level of genetics understanding with US non-majors' pre-test scores, while Indonesian biology majors' scores were two times lower than US majors' post-test scores. The implications of reducing the gap between two countries are discussed.