Organisms frequently encounter change in environment due to this there
will be a change in behavior and physiological change .Genotype some extent
remains constant for environmental change.the phenotypic value is determined
partly by the genotype and partly by the environment experienced by the
individual during the development and expression of the phenotypic value . thus
there are two components parts of the phenotypic value attributed to the
genotype and environmental effects.

             The interaction of genes with external
environment viz climatic, nutritional ,managemental factors is called the
genotype environment interaction in a very simple language.the differential
response of different genotypes in different environment is called genotype
environment interaction .in other words, when a genotype fails to give the same
response in different environment it is taken as    G- E interaction
.

The genes and environment may interact in a number of
different ways:

·        
The environment may affect the genetic
composition of a population by pressure of selection and this leads to
evolutionary changes.

·        
The environment may distort the segregation and
recombination expected on genetic theory.

·        
It may change as mutagenic effects of radiation
and various chemical substances and also as conditioning effect and
paramutation.

·        
The environment may interact with genotypes to
produce differences in phenotypic values of the individuals of a population
this is the most important of immediate concern arising from interplay of
genetic and environmental effects .

 To a statistician and
animal geneticist or breeder the G- E interaction means that the genetic and
environmental variations do not combine their effects additively which means
that the phenotypic value is not simply the sum of the genotype and the
environment but an additional component of variance is associated to phenotypic
value which makes the mathematical model of the phenotypic value as:

                                                           
          P = G + E + IGE

Role of Genotype–Environment Interactions in Breeding and Selection
Programmes

                
        Genotype–environment interactions are usually
evaluated as the relative change in the performance of two or more genotypes in
two or more environments. In many cases it is not sufficient to detect presence
or absence of interactions.There is a need to estimate the magnitude of the
interactions to evaluate their biological significance and role in selection
programmes. Statistically significant interactions may or may not be biologically
relevant, e.g. if they do not affect the ranking order of specific breeds or lines
from one environment to the other.

However, they can be of significant
value if superior individuals such as sires in one environment cannot maintain superiority
in a different environment.Experiments considering different lines,major genes
and their combinations as well as individual sires have revealed significant genotype–environment
interactions in layers and broilers. The magnitudes of these interactions are
different for different traits, genotypes and environments. In layers, significant
interactions are observed with respect to temperate and tropical conditions. In
broilers, the magnitude of such interactions is usually lower, due to limited exposure
to the environmental factors during the short growth period, but there  is evidence of significant genotype–location and
genotype–nutrition interactions. The interactions are usually higher for traits
with lower heritabilities, such as reproduction and feed efficiency, but lower for
traits with higher heritabilities, such as growth and body size. The
unfavourable effects due to genotype–environment interactions on the desired
genotype can be partially overcome by adjusting management conditions to those
required for the optimum performance. However, in many cases such adjustments
are either not possible or not cost effective. In many parts of the world, it is
hard to simulate the optimum environmental conditions for high-yielding
genotypes, since it requires substantial capital investments – for example, to
control the climatic conditions and to provide specific types of cages, a
disease-free environment, or highly concentrated or balanced diets. In many
places, it is also possible that these suboptimal environments may prevail for many
years. Under such circumstances it is more appropriate to select specific
genotypes for specific environmental conditions.

 

                         There are three
important considerations concerning genotype–environment interactions: the
choice of a suitable breed or line; selection for further improvement within
the selected breed or line; and further improvements in the selected genotype through
specific biological effects such as body size or the use of major genes. The
phenomenon of genotype– environment interactions requires additional efforts in
the choice of breeding stock with a general adaptability to more than one environmental
condition. At the same time it offers several opportunities for high production
levels under unfavourable environmental conditions. Appropriate use of this
phenomenon can prove very useful for production of genotypes that are well adapted
and genetically superior for the given environmental conditions.

 

                   Broilers
are marketed at an early agecompared with layers; therefore, they have a
shorter period of exposure to environment and less chance of genotype– environment
interactions. However,there is evidence of significant genotype– environment
interactions in broilers, especially with respect to environmental conditions
such as heat stress and nutrition.Genotype–environment interactions in broilers
with respect to heat stress have been investigated in a series of experiments
at the Hebrew University of Jerusalem, Rehovot, Israel. These investigations
reveal significant interactions of the naked-neck, frizzle and dwarf genes with
ambient temperature Cahaner et al.  studied the effects of normal (25°C) and high
(30°C) ambient temperature on broiler progeny of hens from a sire line and two
dam lines, differing in growth rate and meat yield, carrying the naked-neck (Na)
gene. The advantage of the Nana genotype was much more pronounced at
high ambient temperature in broilers, with genetically higher

growth
rate and breast meat yield. Petek et al. (1999) studied the effect of
genotype–environment interactions on the performance of commercial broilers in western
Turkey. The genotypes were 29 sires and natural climatic conditions in spring and
summer were considered as environments. The interaction was evaluated as
correlations between sire breeding values in summer with those estimated from
their spring offspring. The genotypes that gained more weight in the spring
gained less weight under the hot conditions of summer. The correlation between
the two seasons for weight gain from 0 to 4 weeks of age was 0.26, i.e.
significantly lower than 1. It was even negative, though not significantly lower
than 0, for weight gain from 4 to 7 weeks of age and body weight at 7 weeks of age.
The analysis of variance revealed highly significant genotype–season
interaction effects on all traits. They also observed that this variation was
somewhat related to growth potential.Interactions