Q:

Is the Vulcan ability to control emotions dictated by genetics?

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? 

A:

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.

