This document discusses gene-environment interactions and their role in phenotypic expression and disease development. It explains that all traits are influenced by both genetic and environmental factors, and that variations in genetic makeup are associated with most diseases. Common diseases like diabetes and heart disease are caused by a complex interplay between multiple genetic and environmental risk factors. The effects of genes depend on the environment, and the environment's effects depend on an individual's genes. Several examples are provided to illustrate how changes in environment can influence the phenotype of genetic traits and alter disease risk.

1. Gene environment interaction
Gene–environment interaction (or genotype–environment interaction or G×E) is when two
different genotypes respond to environmental variation in different ways. All traits depend
both on genetic and environmental factors. Heredity and environment interact to produce
their effects. This means that the way genes act depends on the environment in which they
act. In the same way, the effects of environment depend on the genes with which they work.
Nature versus nurture debates assume that variation in a trait is primarily due to either genetic
differences or environmental differences. However, the current scientific opinion holds that
neither genetic differences nor environmental differences are solely responsible for producing
phenotypic variation, and that virtually all traits are influenced by both genetic and
environmental differences. We are all unique. Even monozygotic twins, who are genetically
identical, always have some variation in the way they look and act. This uniqueness is a result
of the interaction between our genetic make-up, inherited from our parents, and
environmental influences from the moment we are conceived.
Gene–environment interactions are studied to gain a better understanding of various
phenomena. In genetic epidemiology, gene-environment interactions are useful for
understanding some diseases. Sometimes, sensitivity to environmental risk factors for a
disease is inherited rather than the disease itself being inherited. Individuals with different
genotypes are affected differently by exposure to the same environmental factors, and thus
gene-environment interactions can result in different disease phenotypes.
Variations in genetic makeup are associatedwith almost all diseases
This is perhaps the most important fact in understanding the role of genetics and environment
in the development of disease. Many people tend to classify the cause of disease as either
genetic or environmental. Indeed, some rare diseases, such as Huntington or Tay Sachs
disease, may be the result of a deficiency of a single gene product, but these diseases
represent a very small proportion of all human disease. Common diseases, such as diabetes or
cancer, are a result of the complex interplay of genetic and environmental factors.
Phenylketonuria (PKU) is a human genetic condition caused by mutations in a gene coding
for a particular liver enzyme. In the absence of this enzyme, an amino acid known as
phenylalanine does not get converted into the next amino acid in a biochemical pathway, and
therefore too much phenylalanine passes into the blood and other tissues. This disturbs brain
development leading to mental retardation and other problems. PKU affects approximately 1
out of every 15,000 infants in the U.S. However, most affected infants do not grow up
impaired because of a standard screening program used in the U.S. and other industrialized
societies. New-borns found to have high levels of phenylalanine in their blood can be put on
a special, phenylalanine-free diet. If they are put on this diet right away and stay on it, these
children avoid the severe effects of PKU. This example shows that a change in environment

2. (lowering Phenylalanine consumption) can affect the phenotype of a particular trait,
demonstrating a gene-environment interaction.
In another example, xeroderma pigmentosum (XP), exposure to ultraviolet light increases the
risk of developing skin cancer in non-carriers of XP mutations, but the combination of these
mutations and exposure to ultraviolet light vastly increases the risk of skin cancer. In theory,
if individuals with XP mutations completely avoid ultraviolet light their risk of skin cancer
becomes close to the background risk.
The common diseases like diabetes and heart disease, are likely due to a combination of
genetic and environmental factors and each contributes a small amount of risk. One affected
person may have more genetic factors, while another has more environmental contributors,
but both reach a threshold that is required for disease onset.
Genetic variations do not cause disease but rather influence a person’s susceptibility to
environmental factors.
We do not inherit a disease state per se. Instead, we inherit a set of a susceptibility factors to
certain effects of environmental factors and therefore inherit a higher risk for certain diseases.
This concept also explains why individuals are differently affected by the same
environmental factors. For example, some health conscious individuals with “acceptable”
cholesterol levels suffer myocardial infarction at age 40. Others individuals seem immune to
heart disease in spite of smoking, poor diet, and obesity. Genetic variations account, at least
in part, for this difference in response to the same environmental factors.
We all carry genetic variants that increase our susceptibility to some diseases. By identifying
and characterizing gene-environment interactions, we have more opportunities to effectively
target intervention strategies. Many of the genetic risk factors for diseases have not been
identified, and the complex interaction of genes with other genes and genes with
environmental factors is not yet understood. Clinical and epidemiological studies are
necessary to further describe these factors and their interactions. However, as our
understanding of genetic variations increases, so should our knowledge of environmental
factors, so that ultimately, genetic information can be used to plan appropriate intervention
strategies for high-risk individuals.
Gene microenvironment
Genes are always operating within the micro-environment of the cell, which is influenced by
the expression of other genes. In addition, an environmental exposure could trigger the
expression of a gene that in turn modifies other genes. The underlying DNA sequence is not
usually altered, but molecular changes occur that affect transcription or translation.
Some environmental stimuli may affect DNA indirectly by altering transcription factors.
These regulatory factors are sensitive to chemical signals like hormones and
neurotransmitters. Thus, if an external event triggers changes in the levels of these signals,
the amount, structure, or activity of transcription factors may also change.

3. Environmental conditions can affect DNA indirectly by modifying epigenetic factors.
Epigenetic factors are compounds that attach to, or "mark" DNA. These factors interact with
genetic material, but do not change the underlying DNA sequence. Instead, they act as
chemical tags, indicating what, where, and when genes should be "turned on" or expressed