“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.”
http://hmg.oxfordjournals.org/content/13/suppl_2/R225.full