Two firefighters, Maria and David, are involved in the same life-threatening building collapse. Both experience significant distress in the immediate aftermath. Six months later, Maria has developed severe and persistent symptoms of Post-Traumatic Stress Disorder (PTSD), while David has largely recovered and shows few lasting symptoms. A research study they participate in reveals that Maria has two short versions of a specific gene involved in regulating serotonin, while David has two long versions of the same gene. Based on this information, explain the most likely biological reason for the difference in their psychological outcomes.