The role of genes in determining characteristics

One of the most dangerous misconceptions about genes is the false belief that genes determine all human characteristics. This thinking can lead to discrimination against people with certain genotypes. It can also discourage people from taking reponsibility for their health and actions, because they incorrectly assume that their choices do not have an important effect.

'Characteristics' can refer to many traits, including appearance, personality, and behavior. The traits an individual exhibits are sometimes referred to as a phenotype. Phenotype is often related to genetic information, or genotype, but genes are not the only determining influences. In a few cases, a single factor, genetic or environmental, can determine an outcome, but the majority of human traits are the product of complex interactions between genes, prenatal conditions, environment, and lifestyle.

Genes - Our bodies are made up of millions of cells, and each cell receives information from the genes in its nucleus. The information encoded in genes guides the structure and function of cells and tissues, as well as how an individual responds to external environmental influences. The effects of genes are very important in guiding early development in an embryo, specifying how organs like the heart and brain develop. The importance of genes in shaping an individual's basic phenotype can be seen in the case of identical twins, who have exactly the same combination of alleles, and who resemble each other very closely. For more detailed information on how traits are inherited through genes, see the section on How genes are inherited.

Prenatal conditions - While the information in our genes is vital to the development of an embryo, there are other factors which affect prenatal growth. A developing fetus' blood supply is linked to that of the mother, so the mother's health can have a great effect on development. In a general example, fetuses of mothers with poor nutrition grow more slowly, because they are undernourished. There are also very specific effects of the prenatal environment on development. Some of these can be positive; for example, folic acid is a vitamin found in many fruits and vegetables which, when consumed by a pregnant mother, has been shown to reduce the risk of certain birth defects. Other prenatal influences can be detrimental; for example, women who consume alcohol while pregnant are at increased risk of having children who are small and who may experience altered mental and physical development. The effects of prenatal environment on infant health are well documented. In addition, there is evidence, not conclusive at this time, that conditions in the womb may have effects on adult health, including the risk of certain conditions, such as schizophrenia and heart disease.

Environment and Lifestyle - There is an age-old debate surrounding the relative contributions of environment and genes to various human characteristics. There are some traits, such as eye color, which seem to be primarily genetically determined. In contrast, many complex emotions and behavior, which may have some genetic component, appear to also be very variable in response to education and environment. We cannot today, nor will we be able to in the near future, change human genes. It is potentially useful to be informed and aware of our genetic predispositions, for example, an elevated risk of diabetes, heart disease, or obesity. However, it is the educational and lifestyle changes we make in response to this information that can have the greatest effect on our health.

Most characteristics, even those that have a clear genetic component, are also responsive to intervention. When there is news reported about a genetic basis for some trait, close examiniation (see the section on Help in Evaluating Medical News) sometimes reveals that the genetic contribution is minor or applies only a specific group of people. The danger of such reports is that they can lead people to overestimate the role of genes in determining a trait, leading to possible discrimination or to a lack of attention to the lifestyle and environmental changes that can work provide significant benefits.

Note: Human genetics is an extremely complex topic. This website is meant only as an introduction and overview. If you are concerned about how genetics may affect your health, always consult your physician.