The Interaction of Genes and Environment

The Interaction of Genes and Environment

Development is the result of an ongoing, bi-directional interchange between heredity and the environment. Gottlieb (1998; 2000; 2002) suggests an analytic framework for the nature and nurture discussion that recognizes the interplay between the environment, behavior, and genetic expression. This bidirectional interplay suggests that the environment can affect the expression of genes just as genetic predispositions can impact a person’s potentials. Likewise, environmental circumstances can trigger symptoms of a genetic disorder. For example, a person predisposed genetically for type 2 diabetes may trigger the disease through poor diet and little exercise.

Each of us represents a unique interaction between our genetic makeup and our environment; range of reaction is one way to describe this interaction. Range of reaction asserts that our genes set the boundaries within which we can operate, and our environment interacts with the genes to determine where in that range we will fall. For example, if an individual’s genetic makeup predisposes her to high levels of intellectual potential and she is reared in a rich, stimulating environment, then she will be more likely to achieve her full potential than if she were raised under conditions of significant deprivation. According to the concept of range of reaction, genes set definite limits on potential, and environment determines how much of that potential is achieved.

How does the environment shape genes?

Canalization

Not all genes are influenced by the environment to the same degree. The degree to which gene expressions are impacted by the environment is referred to as canalization. Some gene expressions are greatly impacted by the environment (shallow canalization), while others produce similar genetic expressions, except under extreme conditions (deep canalization).

One example of a trait that is shallowly canalized is intelligence. Individuals with similar genetic intellectual potential will have different expressions depending on the type of environment in which they grow. Individuals in enhanced environments will likely demonstrate higher intelligence compared to individuals in deprived environments.

One example of a trait that is deeply canalized is reaching motor milestones. Infants around the world will reach the motor milestone of sitting in a wide variety of environments. Only under the most extreme circumstances do infants not learn to sit.

Epigenetics

The developmental psychologist Erik Erikson wrote of an epigenetic principle in his book Identity: Youth and Crisis (1968), encompassing the notion that we develop through an unfolding of our personality in predetermined stages, and that our environment and surrounding culture influence how we progress through these stages. This biological unfolding in relation to our socio-cultural settings is done in stages of psychosocial development, where “progress through each stage is in part determined by our success, or lack of success, in all the previous stages.”

The field of epigenetics looks beyond the genotype itself and studies how the same genotype can be expressed in different ways. In other words, researchers study how the same genotype can lead to very different phenotypes. As mentioned earlier, gene expression is often influenced by environmental context in ways that are not entirely obvious. For instance, identical twins share the same genetic information (identical twins develop from a single fertilized egg that split, so the genetic material is exactly the same in each; in contrast, fraternal twins develop from two different eggs fertilized by different sperm, so the genetic material varies as with non-twin siblings). But even with identical genes, there remains an incredible amount of variability in how gene expression can unfold over the course of each twin’s life. Sometimes, one twin will develop a disease and the other will not. In one example, Tiffany, an identical twin, died from cancer at age 7, but her twin, now 19 years old, has never had cancer. Although these individuals share an identical genotype, their phenotypes differ as a result of how that genetic information is expressed over time. The epigenetic perspective is very different from range of reaction, because here the genotype is not fixed and limited (Tienari et al., 2004).

Video 4.9 Epigenetics explains some of the research that gives insights into the complicated relationship between nature and nurture.

How do genes shape the environment?

Not only does the environment shape our genes, but our the expression of our genes can shape our environments. Sandra Scarr (1992) proposed a theory that explains gene-environment correlation, focusing on how genes interact with the environment. An active gene-environment correlation occurs when ones’ genes influence the environments and experiences that one seeks out. For example, a child who has genes that make them social may seek out after school activities where they get to engage with others.

An evocative gene-environment correlation occurs when one’s genes elicit a certain type of reaction from individuals with whom they engage. For example, a child who has genes that make it difficult to focus may evoke negative attention from their peer and teachers when they are off-task.

A passive genotype-environment correlation occurs when children passively inherit the genes and the environments their family provides. Certain behavioral characteristics, such as being athletically inclined, may run in families. The children have inherited both the genes that would enable success at these activities, and given the environmental encouragement to engage in these actions.