AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

GENOTYPE – ENVIRONMENT INTERACTION

• Under certain combination of genotype and environment, the phenotype may not be equal to the sum of these two variables but rather be smaller or larger.
  • P < or > (G + E)
• When interaction is absent the phenotype equals the sum of genotype and environment.
  • P = (G + E)
• If the ranking order of two (or more) genotypes varies from environment to environment in which they are conducted then there is G × E interaction.
• The best genotype in one environment is not th e best in another environment.
• The G × E interaction is defined as the relative change in the performance of two or more genotypes in two or more environments. That is the phenomenon of genotype – environment interaction is reflected by the differential expression of different genotypes over environments (The genotype can be breeds, strains or lines. The environments can be nutrition, climate, housing and management etc.).
• For example the genotype A may be superior to genotype B in the environment - I, but inferior in environment – II, when G × E interaction is present.

Genotype	Environment - I	Environment - I
A	Superior	Inferior
B	Inferior	Superior

• Therefore each genotype has its specific adaptability for which the G × E interaction is responsible.
• When there is no interaction the best genotype in one environment will be the best in all.
• When the interaction between genotype and environment is present, the phenotypic value becomes P = G + E + I_GE.
• The interaction component also makes changes in the sources of variation for the phenotypic variance and result in V_p = V_G + V_E + V_GE.
• Since the variance occurring in genetically uniform groups is entirely due to environmental differences among the individual, the variance due to interaction is included with environment variance.
• In practical breeding, an important concept concerning G × E interaction is adaptability. In temperate climate, Zebu cattle (Bos indicus) are inferior to the various European breeds of cattle (Bos taurus). In tropical climate, Zebus are superior. The ranking of European breeds and Zebus depend upon the climate in which they are tested. For example the ranking of bulls may vary according to the country in which the performance of their daughters is measured.

Importance of G×E interaction

• Genotype – Environmental (G × E) Interaction are very important if individuals of a population are reared under different conditions, where environment cannot be controlled.
• Experimental evidences shows that the best dairy breed sires in the temperate countries were not the best in the tropical countries.
• The importance of I_GE was also found between countries with a high level of concentrate feeding versus pasture feeding base.
• The best sires in low environment level were not the best in the high environment level.
• Therefore the G × E interaction requires additional efforts in selection of breeding stock with a general adaptability to more than one environment condition or specifically suitable for desired environmental condition.