This article reports on the outcomes of an intervention in a Swedish school in which the author, a teacher-researcher, sought to develop students' (14-15 years old) ethical reasoning in science through the use of peer discussions about socio-scientific issues. Prior to the student discussions various prompts were used to highlight different aspects of the issues. In addition, students were given time to search for further information themselves. Analysis of students' written arguments, from the beginning of the intervention and afterwards, suggests that many students seem to be moving away from their use of everyday language towards using scientific concepts in their arguments. In addition, they moved from considering cloning and "designer babies" solely in terms of the present to considering them in terms of the future. Furthermore, the students started to approach the issues in additional ways using not only consequentialism but also the approaches of virtue ethics, and rights and duties. Students' progression in ethical reasoning could be related to the characteristics of the interactions in peer discussions as students who critically and constructively argued with each other's ideas, and challenged each other's claims, made progress in more aspects of ethical reasoning than students merely using cumulative talk. As such, the work provides valuable indications for the importance of introducing peer discussions and debates about SSIs in connection to biotechnology into the teaching of science in schools.