"No-one has ever identified a stoic gene."
Not yet they haven't. But give them time. It is more likely to be the interaction of several genes anyway or proteins interacting.
"Another factor is the lavish supply of proteins that switch genes on or off, such as the zinc finger family of proteins. This family has expanded independently in humans, yeast, fly and worm but humans still have twice as many zinc finger proteins as the fly and nearly five times as many as the worm. Meanwhile, proteins themselves can be modified, for example by enzymes snipping bits off them, or by the addition of sugars or fats to change their activity.
This builds into a picture of exquisite layers of control of genes and proteins, with genes being turned on and off, up and down, with extraordinary subtlety driving our development from fertilized egg to adult, and maintaining and repairing our bodies during the rigours of daily life."
http://www.wellcome.ac.uk/en/genome/thegenome/hg01f006.html