Study 1 - Caspi et al (2003) - 5-HTT gene and the influence of life stress on depression
The causes of depression are numerous, and vary a great deal from person to person. However, given that there is a tendency for depression to run in families (using research methods such as family and twin studies as described above), there has long been the suspicion that at least a part of the cause of depression is genetic. Once this question has been answered, of course, the next question is which genes might be responsible? Again, it is highly unlikely that such a complex and varying condition will only have one gene associated with it, but researchers have begun to identify some genes which could play a role in the process. Caspi et al (2003) was one of these studies, looking at the effect of possessing different types (alleles) of the 5-HTT gene.
Genes and alleles... a recap!Just in case you've forgotten your GCSE Biology... here is a reminder of a few key terms for this study.
Gene - a length of DNA which codes for the production of a protein
We have two sets of each gene in our bodies (one from each parent). Different versions of the same gene are called alleles (so for example, in the genes which code for eye colour one allele may code for brown eyes, and another allele may code for blue eyes).
5-HTT alleles - long and short
In the 5-HTT gene, the two alleles which were studied by Caspi were known as the 'long' and 'short' alleles. In the human population, the frequency of the long allele of 5-HTT is about 57% that of the short allele is 43% (OMIM, 2003). These alleles coded for a protein which acts as a serotonin transporter (responsible for taking serotonin back up from the synapse into the neuron). See the picture below for a more detailed explanation. The 'long' allele was thought to lead to the production of a greater number of serotonin transporters, giving greater control over the levels of serotonin in the synapse. Evaluating Caspi et alCaspi et al provide strong evidence for the need to look at the interaction of nature and nurture (or of genes and the environment) in examining human behaviour. However, there is some debate about exactly what their results seem to show.
Genes and alleles... a recap!Just in case you've forgotten your GCSE Biology... here is a reminder of a few key terms for this study.
Gene - a length of DNA which codes for the production of a protein
We have two sets of each gene in our bodies (one from each parent). Different versions of the same gene are called alleles (so for example, in the genes which code for eye colour one allele may code for brown eyes, and another allele may code for blue eyes).
5-HTT alleles - long and short
In the 5-HTT gene, the two alleles which were studied by Caspi were known as the 'long' and 'short' alleles. In the human population, the frequency of the long allele of 5-HTT is about 57% that of the short allele is 43% (OMIM, 2003). These alleles coded for a protein which acts as a serotonin transporter (responsible for taking serotonin back up from the synapse into the neuron). See the picture below for a more detailed explanation. The 'long' allele was thought to lead to the production of a greater number of serotonin transporters, giving greater control over the levels of serotonin in the synapse. Evaluating Caspi et alCaspi et al provide strong evidence for the need to look at the interaction of nature and nurture (or of genes and the environment) in examining human behaviour. However, there is some debate about exactly what their results seem to show.
- On the one hand, this seems to be clear evidence of a genetic factor in the development of depression. People with short alleles were the most likely to become depressed after stressful life events.
- On the other hand, since so many people carry the s allele (almost half the population), it is hard to say that in is the gene that made the difference. Maybe it was the environmental factors (stressful events) that caused the depression. Certainly there is not a cause-effect relationship between the genes and the condition.
- There are some potential ethical problems with studies like this (see the 'Ethics of Genetic Research' page for more)