I've seen a few Vulcans lose control of their emotions in various Star Trek media. Primarily Spock, who is in fact half-Vulcan.
I was under the impression that the ability to control emotions was down to training and ritual. Is it also linked to genetics?
From the Memory Alpha article on Vulcans, The Vulcan brain has the ability to physically modify itself:
Unlike most humanoid species, traumatic memories were not only psychologically disturbing to Vulcans, but had physical consequences as well. The Vulcan brain, in reordering neural pathways, could literally lobotomize itself. (VOY: "Flashback")
Vulcans learned to gain conscious control of many of these functions, allowing them to regulate their bodies to a high degree by simple will power. When injured a Vulcan could go into a trance-like state, using this ability to concentrate all of his or her energy onto repairing the injury. (TOS: "A Private Little War")
Knowing that, here's some extra information from the page on Emotions:
Since Vulcans have the capability to exert a level of conscious control over virtually all of their body functions, they can actually manipulate the neural pathways that regulate and receive the balances of neurochemicals involved in emotion. This enables them to inhibit their brain from forming the neural impulses that create conscious emotional mental states, thereby actually suppressing their innate emotions. The psycho-suppression system in the Vulcan brain that endows Vulcans with this ability is located in the mesiofrontal cortex. (VOY: "Meld") The Vulcan ritual, known as the kolinahr, is designed to purge all remaining emotions in a Vulcan. (Star Trek: The Motion Picture)
So no, the Vulcan ability to control emotions is not dictated by their genetics, but it is supported by their genetics. This is shown in VOY 3x14, Alter Ego, when Tuvok makes some progress in teaching Harry Kim to control his emotions.
