This paper reports on the genetic and environmental bases of differences in reading and spelling in a large unselected adolescent twin sample. The dual route cascaded (DRC) model of reading is introduced and used to provide endophenotypes for the basic psychological functions underlying reading. The model is then extended to a complementary dual-route model for spelling with tests tapping lexical and sublexical routes to spelling. Univariate analyses of the DRC-based phenotypes are reported. Model fitting supported high levels of additive genetic control over learning to read (heritabilities of 0.73, 0.71, and 0.61 for irregular, nonword, and regular word reading respectively), with remaining variance being best modelled as unique environment (including measurement error) rather than differences between families. Again, within the power of the present design, models specifying sex-limited genetic mechanisms were not supported, suggesting that the genes for both routes to reading are the same for both sexes and control similar proportions of variance. For spelling, the heritabilities were similar for the tasks corresponding to lexical and nonlexical reading (h2 = 0.76, 0.52 respectively). The value of future multivariate analyses for testing predictions from competing psychological models of reading are discussed, as is the need for linkage and association studies and for the development of more basic endophenotypes to fully resolve the genetic architecture of this important human behaviour.