The reaction norm corresponds to the genetically designed range of possible variations of two phenotypes of the same genetic material. The ultimate trait expression within this predetermined bandwidth depends on the external environmental influences in each case. The range of modification also plays a role in the context of genetic predispositions to disease, which need not automatically lead to actual disease.
What is the reaction norm?
The extent of the ability to modify lies as a reaction norm in the genes themselves. Thus, the genetic reaction norm is the specific range of variation in the phenotype given the same genotype. The genotype is the hereditary image of an organism and is considered to represent the genetic makeup and thus the framework of the phenotype. Thus, the genotype determines the possible range of morphological physiological trait expression in the phenotype. However, due to the principle of phenotypic variation, significant differences in individual traits can occur despite the same species membership. Phenotypic variation forms the basis for evolutionary change. Even with the exact same genotype, phenotypic variation is not excluded. Thus, identical twins with 100 percent identical genetic material may correspond to different phenotypes to some extent. Phenotypic variation in the same genotype should be understood as a response to environmental influences. Genetically identical organisms develop many different traits when exposed to different types of environmental stimuli and thus differ in appearance. Changes in phenotype caused exclusively by environmental influences, and thus without gene difference, are adaptive responses, also known as modification. The extent of the modification ability lies as reaction norm in the genes themselves. Thus, the genetic reaction norm is the specific range of variation in the phenotype for the same genotype. The term reaction norm goes back to Richard Woltereck, who first used it in the early 20th century. The term breadth of modification is considered synonymous.
Function and task
Despite having exactly the same genetic material, identical twins can differ from each other to a greater or lesser extent when they grow up in different environments. The range of these differences is defined in the reaction norm. For example, individuals of the same genotype do not have to be exactly the same size. Their reaction norm specifies a spectrum within which their size can range. For example, this spectrum might provide for a minimum of 1.60 meters and a maximum of 1.90 meters. What size individuals actually develop depends on their environment. This reaction to environmental conditions is thus genetically inherent with the modification range. Thus, the principle of natural selection affects the reaction norm. In the case of extremely variable environmental conditions, a greater variability is required. Thus, in an environment with high variability, a relatively broad reaction norm promises higher survivability. In niches with relatively unchanging environmental factors, only a narrowly defined reaction norm can be expected for individuals of the same genetics, since high variability is not particularly worthwhile for the goal of survival when environmental factors remain constant. Plants of the same genotype are capable of developing different leaf shapes depending on their location, for example. In the sun, they develop harder and smaller sun leaves. In the shade, on the other hand, they develop thinner shade leaves. In the same way, many animals are able to change their coat color depending on the season. For humans, this also means that their genes provide them with different possibilities for their physique. Which of these possibilities are ultimately retrieved depends heavily on the experiences to which each individual is exposed, or will be exposed. The response norm ultimately depends on the ecological niche. That is, the milieu and variability of the environment determines how broad the phenotypic expression of individuals must be in order for them to have an evolutionary advantage. Actual expression only sets in with the presence or absence of a particular environmental influence.
Diseases and disorders
Basically, modifications are to be distinguished from mutations.Phenotypic modifications occur within the genetic response norm, but are not inherited automatically or fixed. For example, if a hare changes coat color to white in winter, it will not give birth to pure white bunnies. However, its offspring can change coat color again within the inherited range of modification depending on environmental influences. The reaction norm adapts to changing environments on a genetic basis to the extent that it can become narrower or broader over time depending on a change in variability of a particular environment niche. In the case of a permanent absence of snow over decades or even centuries, a hare will no longer benefit from a modification range of its coat color for survival in the given niche. Thus, the reaction norm may genetically narrow. Clinically, the reaction norm is most relevant in the context of genetic dispositions. An individual with a genetic disposition for a certain disease carries a higher risk of disease onset, which is inherent in his genes. However, the increased risk does not necessarily lead to actual disease. For example, if two identical twins carry the same genetic predisposition to cancer, both individuals will not necessarily develop cancer during their lifetime. Assuming they followed the exact same lifestyle, they would both either get the disease or not get the disease. However, if they follow a different lifestyle with different stimulus exposures, this may result in the disease of one of the individuals. In connection with external influences on disease, medicine speaks of exogenous factors. Genetic disposition for a disease is an endogenous factor. Despite the endogenous disposition, the targeted avoidance of disease-causing exogenous factors can, under certain circumstances, prevent the genetically predisposed disease. These correlations are ultimately a result of the response norm or modification band. If they did not exist, the onset of disease would be determined solely by endogenous factors and thus would be genetically certain to be preprogrammed.
