Using the Biochemical Pathway Model to Teach the Concepts of Gene Interaction & Epistasis

Typical dihybrid Mendelian phenotypic ratios are 9:3:3:1 for AaBb X Aabb crosses. If the genes involved interact in their effects on the phenotype, altered phenotypic ratios such as 9:3:4, 9:7, 12:3:1 and others may result as a consequence of gene interactions called epistasis. The colors of onions found in grocery stores are a relevant example. When red onions (AABB) are crossed with white onions (aabb), the dihybrid offspring are red (AaBb), and the offspring of a mating between dihybrids (AaBb x AaBb) are 9 red: 3 yellow: 4 white onions. Students learning genetics often have difficulty grasping the concept of gene interaction well enough for them to be comfortable predicting the outcomes of crosses or interpreting the genetic bases of such outcomes. A method I use successfully to teach gene interaction is to give examples where the genes involved affect various steps in biochemical pathways. A biochemical (metabolic) pathway is a sequential series of chemical reactions by which substrates are altered step-by-step to produce the desired end product. Once students grasp the notion that genes can interact because they control different steps within biochemical pathways, the reason why phenotypic ratios appear to be non-Mendelian becomes more obvious and comprehensible.