Genetics is an area that is increasingly important in people's daily lives. Despite this, its teaching/learning is among the most challenging topics in biology. Several instruments were created over a number of years to ascertain the quality of teaching and learning genetics. In our study, we created a specific instrument to investigate Mendelian inheritance, trying to verify, in addition to the students' level of knowledge, their certainties about their answers and their seriousness responding to the instrument. Our results demonstrate that most of the previously reported students' difficulties persist, even with years of research pointing out problems in teaching/learning genetics. More seriously, a large number of students are convinced about their knowledge, even if they are erroneous, which may mean that the errors presented are not only due to forgetfulness or lack of hardness in answering the instrument, but rather to erroneous learning. The data collected will be used to help improve the teaching of this topic of genetics through the creation of teaching materials and workshops for teachers.