Findings from a study of 10 German students aged 15-19, using problem-centred interviews, suggest that many students hold an 'everyday' conception of genes as small, trait-bearing, particles. Analysis of this notion identified a number of ways in which such a view might restrict the ability of students to develop an understanding of the scientific explanation. For example, if genes are equated with trait there is no clear distinction between genotype and phenotype, and hence little need to consider a mechanism by which a gene could be expressed in the phenotype. This everyday perspective provided a plausible explanation of the difficulties and misconceptions found, after formal teaching of genetics, in a survey of 482 English students aged 14-16 based on written questions and interviews. Drawing on this analysis, an approach to teaching genetics and inheritance that takes account of students' everyday views is suggested.