Extensions of Mendelian Genetics

Beyond simple monohybrid, dihybrid and trihybrid crosses, real inheritance often produces phenotypic ratios that do not match the clean 3:1, 9:3:3:1 or 27:9:9:9:3:3:3:1 patterns - exceptions to strict Mendelian expectation known collectively as the extensions of Mendelian genetics. These exceptions arise from gene interaction: a single phenotype controlled not by one gene pair acting alone but by the combined action of two or more genes, each with two or more alleles. W. Bateson introduced and explained this concept, which is accordingly also called the Factor hypothesis or Bateson's factor hypothesis. Bateson classified gene interactions into two broad categories based on where the interacting alleles sit: intragenic (intra-allelic) interactions, between the alleles of the very same gene at one locus - covering incomplete dominance, codominance, multiple alleles and pleiotropy - and intergenic (non-allelic) interactions, between alleles of different genes at different loci - c …