Mendel’s laws of heredity on his 200th birthday: What have we learned by considering exceptions?

Violations of Mendel’s laws can generically be referred to as ‘non-Mendelian inheritance’. However, from that broad perspective, nearly all inheritance systems would show non-Mendelian inheritance (at least to some degree). To hold exactly, Mendel’s laws impose strict requirements: a locus has to contain two allelic variants that have discrete effects on categorical (or at least discrete and countable) traits, and they must show complete dominance. These strict conditions are rarely met in real systems (Hou et al. 2016), both because allelic effects do not adhere to the strict law of dominance and because many traits of interest show continuous variation. Mendel recognised many of the exceptions related to effects of alleles, such as the presence of incomplete dominance, pleiotropy, and epistasis (see Fairbanks 2022, this volume), and Fisher (1918) reconciled the assumption of Mendelian inheritance with continuous variation. Hence, from this perspective, a large array of scenarios that show nonMendelian inheritance are actually consistent with the conceptual foundation of Mendel’s perspective based on elemental inheritance