Single gene disorders with Mendelian inheritance patterns have contributed greatly to the identification of genes and pathways implicated in genetic disease. In these cases, molecular analysis predicts disease status relatively directly. However, there are many abnormalities which show familial recurrence and have a clear genetic component, but do not show regular Mendelian segregation patterns. Defining the causative gene for non-Mendelian diseases is more difficult, and even when the underlying gene is known, there is uncertainty for prenatal prediction. However, detailed examination of the different mechanisms that underlie non-Mendelian segregation provides insight into the types of interaction that regulate more complex disease genetics.