Summary. This paper presents the results of a study talking to children and young people affected with severe haemophilia A and/or haemophilia B about their knowledge and understanding of genetics and inheritance. These data were gathered in a qualitative study using semi-structured interviews with thirty boys aged four to sixteen discussing the impact of haemophilia on their lives. Responses were tape recorded, transcribed and analysed, using thematic analysis; one of the themes identified was genetic knowledge which is presented in this paper. Genetic knowledge was formed within the context of normal day-to-day lives within families affected by haemophilia, with parents and haemophilia centre staff being sources of information about individual inheritance patterns as well as providers of information about the future genetic impact of having haemophilia.