In one type of digenic example, mutants are homozygous at both of two homologous PCP genes with functional redundancy (e.g., Dvl1;Dvl2 or Fzd3;Fzd6) and nearly all embryos have craniorachischisis; homozygotes for any one of these genes do not. The gene discussed is DVL1; the disease is craniorachischisis.