Look at a collection of family photographs, and you can see the influence of genes on shared physical characteristics such as eye color and the shape of facial features. Genes are also a known factor in certain illnesses, including heart disease and some cancers. But when it comes to personality and behavior, the role played by our genetic inheritance is rather more difficult to unravel.
Genes are found inside the nucleus of every cell. They are the set of instructions for making enzymes –the proteins that enable cells to function properly. Although humans mostly inherit the same basic set of genes, the human genome, there are small variations in the structure of each gene. These inherited variations can result in considerable differences in the way the brains of different individuals function because specific enzymes are needed for the production of neurotransmitters (messenger molecules by which brain cells communicate).
Inheriting characteristics
While many different influences throughout life shape personality, the biological component of personality – known as temperament – is present from birth and is likely to be influenced by genes. If you have children, you probably find that they seem to develop unique personalities from a very early age despite growing up in a very similar environment. But how much of an adult’s personality is determined by genes?
Physical characteristics, such as eye color, are each determined by several genes, and the same is true of personality characteristics. So, it is not simply a matter of inheriting a single gene ‘ for ‘ a particular characteristic. In addition, for behavioral characteristics, environmental factors such as parenting, education, lifestyle, and diet will probably have at least as strong an influence as genetic inheritance. For example, a person who inherits genes that make them susceptible to depression may indeed become depressed. Still, the influence of a stable family and friends, a good education, and a fulfilling career may well protect the person. Similarly, someone who inherits genes that predispose them to antisocial behavior might get into trouble in an environment ·with opportunities for crime but might not be in another environment with more positive influences. In short, genes can predispose individuals to develop particular behavioral traits or psychological disorders, but whether they do go on to develop them is far from certain.
When the ‘working draft’ of the human genome was unveiled in 2001, scientists were surprised to learn that humans have less than a quarter of the 140,000 genes originally estimated, and the implications of this finding are still being considered. The low number of genes in the genome suggests that the role of environmental factors in behavior may be even greater than previously thought. There simply aren’t enough genes acting in the brain to determine the full range of human behavior in all its subtlety and complexity. Instead, a relatively limited set of genes is interacting with myriad environmental influences.
Genes and destiny
While inherited temperament can play a major part in mental and physical health, there are probably as many ways that you can make your inherited behavioral characteristics work for you as against you. If you are born with genes that make you susceptible to stress, for example, you might take this into account when choosing a career. The thing to remember is that while genes may have a powerful influence on behavior and health, they are by no means the whole story; lifestyle and environment are just as important.
Identical Twin Studies
How do experts assess the relative influence of genes and environment on any particular characteristic? It’s tricky, but estimates can be made by studying pairs of identical and fraternal (non-identical) twins. In the table below, the figures in each column show the ‘concordance’ – that is, the likelihood of both twins sharing that characteristic. Identical twins have twice as many genes in common as fraternal twins, but both types of twins share environmental factors to the same extent. So, comparing the figures in the two columns helps to separate genetic and environmental factors: the greater the difference between the concordance figure for identical and fraternal twins, the more likely it is that the character has a strong genetic component. So here, hair and eye color are strongly genetic, while measles and breast cancer are environmentally determined.
