Non-Mendelian genetics.

DNA sequence variation potentially can influence disease risk through mechanisms that do not follow Mendel's laws. Non-Mendelian mechanisms include mitochondrial DNA variation, de novo variation, and parental and parent-of-origin effects. To date, these mechanisms have been primarily examined in either rare conditions or a small subset of common conditions. Determining whether non-Mendelian effects account for a measurable proportion of common, complex diseases will require different study designs or additional DNA sources than have been used to identify effects of variants presumably acquired through Mendelian transmission. Epigenetic factors - heritable characteristics of chromosomes other than DNA sequence variation that influence gene expression - account for non-Mendelian effects in some rare conditions but their role in common, complex phenotypes is just beginning to be explored.