Final answer:
The serotonin transporter gene has long and short alleles that can influence an individual's vulnerability to depression and PTSD in the presence of stressful life events.
Step-by-step explanation:
The serotonin transporter gene (5-HTT) has a length polymorphism that affects the level of serotonin availability in the brain. The different alleles of the gene are referred to as long (l) and short (s) alleles. A study by Caspi et al. (2003) found that individuals with the short allele of the gene who experienced stressful life events were more likely to develop depression compared to individuals with the long allele.
Another study by Xie et al. (2009) found that the short allele of the serotonin transporter gene interacted with stressful life events to increase the risk of developing posttraumatic stress disorder (PTSD).
These studies suggest that the serotonin transporter gene, specifically the long and short alleles, can influence an individual's vulnerability to depression and PTSD in the presence of stressful life events.