© CAB International, 2020. Quantitative Genetics, Genomics and Plant Breeding, 140 2nd Edition (ed. M.S. Kang) Introduction Genotype–environment interaction (GEI) is an age-old, universal issue that relates to all living organisms (Kang, 1998). Genotypes and environ- ments interact to produce an array of phenotypes. GEI is the variation caused by the joint effects of genotypes and environments (Dickerson, 1962). Baker (1988) defned GEI as the difference be- tween the phenotypic value and the value ex- pected from the corresponding genotypic and environmental values. When responses of two genotypes to different levels of environmental stress are compared, an interaction is described statistically as the failure of the two response curves to be parallel (Baker, 1988). Recently, de Leon et al. (2016) defned GEI as the differential sensitivity of certain genotypes to different en- vironments. The GEI issue is not only import- ant in plant-breeding programmes but also in animal-breeding programmes (Lin and Lin, 1994; Montaldo, 2001; Hayes et al., 2016). Genotype-by-environment interaction must be distinguished from phenotypic plasticity. Schlichting (1986) defned phenotypic plasticity as the ability of an individual organism to alter its physiology/morphology in response to changes in environmental conditions. It describes the range of phenotypes produced by a single genotype in different environments (Grogan et al., 2016). The different phenotypes are referred to as norms of reaction (Redei, 1982; Kang, 1998). Pheno- typic plasticity is a useful mechanism in plants, as being immobile, plants, unlike animals, can- not move away from a stressful situation but can adjust their performance through phenotypic plasticity. For GEI to occur, there must be at least two genotypes (df = 1) tested in at least two en- vironments (df = 1), which will yield GEI with 1 df (Kang, 1998). van Eeuwijk et al. (2016) con- sider the GEI problem as the building of predict- ive models for genotype-specifc reaction norms. We should also distinguish between GEI and genotype–environment correlation (covari- ance) (GEC). GEC occurs when phenotypic and environmental effects are not independent. When additional agronomic inputs are given or are necessary for certain genotypes to do well, a genotype-by-environment correlation is created (Kang, 1998). Interaction or GEI occurs when the difference between the average phenotypic value for two genotypes changes in different en- vironments, but GEC refers to the situation when particular genotypes tend to be associated with positive, and others with negative, environmen- tal effects (Crow, 1986; Doolittle, 1987). 9 Genotype–Environment Interaction and Stability Analyses: An Update Manjit S. Kang* Kansas State University, Manhattan, Kansas, USA * Email: manjit675264@gmail.com © CAB International NOT FOR RESALE.