Characteristics that are influenced by environmental as well as genetic factors are called multifactorial. The idea of “nature versus nurture” — in other words, the relative influence of genetics versus environmental factors — has been and still is debated. Just looking at the genes of a given organism will not determine how that organism will develop and act. Even identical twins will show different characteristics, depending on the environment in which they live. Everyone is a product of their environment as well as their genetics.
Even when influenced by the environment, phenotypes have a normal range of expression. For instance, human height varies based on nutrition and genetics, but not many people are shorter than 4½ feet or taller than 7 feet. The range of phenotypic possibilities is called the norm of reaction. Hydrangeas, for example, may be blue, pink, or purple, but they are never naturally orange. Hydrangeas are blue in acidic soil with available aluminum, and they are pink in alkaline soil without available aluminum.
Some human characteristics have a narrow norm of reaction, such as blood type. Others have a wide norm of reaction, such as the number of blood cells in humans, which varies depending on factors that include physical fitness, presence or history of infections, and even the altitude at which a person lives.
The environment also affects human genes. Serotonin, a neurotransmitter that acts inside brain cells, lowers anxiety and depression during traumatic times. Mutations in the serotonin transporter gene may cause a reduced ability to cope with stress. That does not mean that the person is always depressed, but if the environment produces stress, the person may become depressed more easily than a person with unmutated serotonin transporter genes.
You already learned about PKU, a pleiotropic disorder caused by defects in a single gene coding for an enzyme that converts the amino acid phenylalanine to tyrosine. Newborns are tested for this defect very early in life (Figure 1), so that if the results are positive, they can be given a diet limiting phenylalanine ingestion. That way, the toxic buildup is prevented and the children can develop normally. PKU is an example in which environmental factors can modify gene expression.
Two identical twins (female) live in different parts of the country. One is very committed to a healthy lifestyle: not smoking, exercising regularly, eating a diet rich in fresh produce, and avoiding red meats and processed foods. The other is not as careful: she smokes, is overweight, and often eats fast and processed foods. They are aware that several women in their family have had breast cancer, and decide to consult a doctor about their odds of developing the disease. Which of the following statements by the doctor sounds most correct?
- As identical twins, you are genetically the same, so your chances of developing breast cancer are identical.
- The twin with the healthy lifestyle should not be terribly concerned, while the one with the unhealthy lifestyle is at a higher risk.
- Breast cancer has a genetic component, and the twins have identical genes, so they have the same genetic risk. However, environmental factors such as smoking, obesity, and consumption of red meat have been shown to increase the risk of cancer. While both twins should monitor themselves closely, the twin who smokes and is overweight may want to consider a healthier lifestyle to decrease her risk of breast cancer.
While genes and genetic causes play a large role in health and phenotypes, the environment also plays an important role. Understanding this can enable the treatment of some disorders, such as the case with PKU in which limiting the intake of phenylalanine can prevent toxic build up of this amino acid. Often the norm of reaction is set by genetic factors but ultimately determined by environmental exposures.