1.7 Familial inheritance and heritability in humans
Temperamental characteristics and mental disorders frequently run in families. Thus the close blood relatives (children, siblings and parents) of patients with major depression or bipolar disorder are much more likely to suffer from these conditions than people from the general population.
How can it be ascertained what contribution genetic factors make to such disorders? One way is to look at what combinations of genetic factors make it more likely that a person will be vulnerable to environmental factors such as stressful events, and hence to developing disorders such as depression. This approach is discussed later, in Section 3.1.
Another way is to look in detail at precisely how genetic factors act on the brain and body to make a disorder more likely.
Here we restrict ourselves to considering, briefly, the issue of heritability (Box 1), which attempts to give a numeric value to the contribution genetic factors make to the development of particular traits or mental health conditions.
Box 1 Heritability
Heritability is a measure of how much of the variation between individuals in a given character is due to differences in their genes, rather than to differences in their environments, in a particular population. It is expressed as a number between 0 (definitely not due to differences in genes) and 1 (wholly due to differences in genes). It can also be expressed as a percentage, from 0% to 100%. Note that a heritability of 0.4 for a disorder such as depression does not mean that 40% of cases of depression are caused by genes, or even that a specific individual’s depression is 40% due to genes. Every case is caused by genes and the environment in combination. The heritability figure of 0.4 means that, within the study population, 40% of the variation in whether people get depression or not is due to differences in their genes.
Trying to put a figure on genetic contributions to characteristics that run in families is, of course, complicated by the fact that inheritance in families can arise from social learning or culture. For instance, the children of Christians tend to be Christians, while those of Muslims tend to be Muslims, but the inheritance of religious affiliation is clearly sociocultural rather than genetic, so in this case heritability is 0 (or 0%) (Box 1).
In many other situations the relative contribution of genes and environment is much less obvious. Epidemiologists who are interested in the genetic basis of disorders have a number of strategies to overcome this difficulty. One important approach is to look at the incidence of a disorder amongst sets of identical twins. Identical twins inherit the same genes from their parents, so any differences between them are likely to be due to environmental effects. Adoption studies involving identical twins have proved invaluable in disentangling genetic and environmental influences.
Activity 4 Characters of identical twins
Imagine that a study of identical twins adopted into very different family environments at birth showed that as adults: (i) they were very similar in character X; (ii) they were very different in character Y. Explain, with reasons, what this suggests about the heritability of characters X and Y.
The environment in which the identical twins were raised was very different but their genetic inheritance was the same, so (i) suggests that genes had most impact on the development of character X (i.e. X has high heritability); (ii) suggests that the environment had most impact on the development of character Y (i.e. Y has low heritability).
Using approaches such as these, genetic epidemiologists have estimated that the heritability of major depression is 31%–42% (Sullivan et al., 2000). The heritabilities of anxiety disorders such as GAD, OCD, specific phobias and panic disorder have a similar range, from 30% to 40% (Smoller et al., 2008). For comparison, the heritabilities of schizophrenia and bipolar disorder are estimated to be 50%–70%.
Their heritability values suggest that both major depression and anxiety disorders are multicausal, since both genetic and environmental factors make substantial contributions. Genetic influences, and the interaction of genetic and environmental factors, will be considered further in more detail later in this course.
But first we will look in more detail at the biological stress response.