Rodents

By studying genetically controlled rodents in controlled environments, it is possible to discover whether a phenomenon is determined by genes, the environment, or both. For example, assume an investigator wanted to discover whether genetic differences or exposure to some environmental factor causes cataracts in mice. In this example, the investigator suspects that the cause is bright light, but he has also noticed that not all of his animals get cataracts. To further complicate matters, the animals that do get cataracts get them at different ages. To test whether environmental factors affect cataract formation, this investigator can place genetically identical mice in different parts of the animal room, near to lights and further away from lights. Differences in number of cataracts formed and the age at which they are formed must be due to environmental differences, since the animals are genetically identical. In this case, he will find that animals on the top of the cage rack will develop more cataracts sooner than animals further from the ceiling lights in the room. To determine whether genes affect cataract formation the investigator can place mice of different strains in the same environment. By putting mice of more than one strain close to the lights he will find that mice of some strains are more susceptible to cataract formation than are others. Thus, he will have shown that both the environment and genes are important in the development of cataracts.

While many, perhaps most, variables are affected by gene-environment interaction, the relative contributions of genes and environment differs from one variable to another. Eye color is largely determined by genes. Skin wrinkling is largely determined by exposure to sunlight (U.V. light). One of the objectives of research is to find the genetic and environmental factors that underlie age changes and age-related diseases in order to be able to improve the health of individuals and allow them to reach their maximum potential life span.

The genotype of the individual (or inbred strain) is the total set of genes that animal carries. The phenotype of the individual is the set of characteristics that the animal manifests, and is the result of the genotype and its interaction with the environment in which it is expressed. Most such interactions are complex and not as simple as hair color or eye color. Genes interact with one another and with environmental variables. Single traits, such as height or body weight, may be the result of several genes and their interaction with nutritional variables, activity level, and room temperature. Very complex traits, such as depression, anxiety, emotionality, intelligence, and longevity, are undoubtedly the result of the interactions of many, many genes and many environmental factors as well.