Genetic models of ASD risk. Schematic representations of Mendelian and polygenic models of ASD risk are depicted, with evidence for and against each model listed below. In the diagram at the top, the rows represent the type of individual: those with ASD, and those with some risk factors but not sufficient to manifest the clinical syndrome, such as unaffected relatives. The columns represent the basic categories of genetic models under consideration. The size of the variant represents effect size, with a larger symbol indicating increased effect size. For simplicity, these models are presented as distinct categories, whereas in reality ASD risk is likely to be represented by a more continuous distribution of risk architecture. A single asterisk indicates that there is evidence to suggest that de novo CNVs in unaffected controls are smaller [21, 51, 52] and less gene-rich [20, 21] than in people with ASD. A double asterisk indicates that there is conflicting evidence for increased oligogenic heterozygosity [25, 156].