An article in today's New York Times dramatically illustrates how personality is genetic:
On an animal-breeding farm in Siberia are cages housing two colonies of rats. In one colony, the rats have been bred for tameness in the hope of mimicking the mysterious process by which Neolithic farmers first domesticated an animal still kept today. When a visitor enters the room where the tame rats are kept, they poke their snouts through the bars to be petted.
The other colony of rats has been bred from exactly the same stock, but for aggressiveness instead. These animals are ferocious. When a visitor appears, the rats hurl themselves screaming toward their bars.
“Imagine the most evil supervillain and the nicest, sweetest cartoon animal, and that’s what these two strains of rat are like,” said Tecumseh Fitch, an animal behavior expert at the University of St. Andrews in Scotland who several years ago visited the rats....
Of course the article is talking about animal personalities and not human personalities, but the breeding experiments work on several different species of animal, so it goes to follow that if humans were bred for personality characteristics we would see just as dramatic a change in a few generations. Humans aren't so much different from animals as some would like to think.
The other point of interest is that when animals are bred for tameness, their appearance changes:
... other changes appeared along with the tameness, even though they had not been selected for. The tame silver foxes had begun to show white patches on their fur, floppy ears, rolled tails and smaller skulls.
This is proof that that genes which affect brain function also affect other physical features.
Earlier today I blogged about the theory that some genes which cause higher intelligence also cause lower athleticism. The animal experiments support the hypothesis that different personalities or levels of intelligence lead to a different physical appearance in humans.