To make meaning of scientific knowledge in such a way that concepts and values of the life-world are not threatened is difficult for students and laymen. Ethics and morals pertaining to the use of genetic tests for hereditary diseases have been investigated and discussed by educators, anthropologists, medical doctors and philosophers giving, at least in part, diverging results. This study investigates how students explain and understand their argumentation about dilemmas concerning gene testing for the purpose to reduce hereditary diseases. Thirteen students were interviewed about their views on this issue. Qualitative analysis was done primarily by relating students' argumentation to their movements between ethics and morals as opposing poles. Students used either objective or subjective knowledge but had difficulties to integrate them. They tried to negotiate ethic arguments using utilitarian motives and medical knowledge with sympathy or irrational and personal arguments. They discussed the embryo's moral status to decide if it was replaceable in a social group or not. The educational implications of the students' use of knowledge in personal arguments are discussed.