Woo val breeding!
A long time ago I wrote a "assuming vals worked via Mendelian Genetics" musing which seems to reflect how val breeding works.
Note: I acknowledge that this model is my own theory, and both this level of detail is neither necessary for the story or takes into account wibblywobblymagic stuff. It is meant to be educational and give some fun grease for story wheels.
By a Mendelian model, the "Val Gene" (V) has to be a dominant gene, with all other humans having the recessive form (v). Whenever to people make a baby--except in rare circumstances--each parent donates 1 copy of a particular gene. If the dominant V gene is inherited, the child is a val. If no V is inherited, the child is a human (well, a non-val human, anyway). Because V is dominant, as long as one copy is inherited, the val trait is inherited.
All vals would have one of two genotypes: VV (known as homozygous dominant), or Vv (known as heterozygous). In both cases, they have at least on V so they are a val.
If you have two Vv vals breeding, however, there is a 25% chance that the child will be vv, and be human. Because this human lacks the V gene form (dominant), it is impossible for val heritage to reemerge from their line. This chance grows higher if you have a Vv val and a vv human, which means you have a 50% chance of breeding a non-val human.
Assuming they also work by Mandelian genetics (they don't seem to, but lets try!), dark-kinism would have to be a recessive gene (d) vs. Normal human (D). This is not the same gene as the Val Gene, with the val gene possibly overriding the effects of D and d.
If recessive, you only express dark-kinism if you have dd, while most humans would be DD . If you have a dark-kin breed with a normal DD human, all children are human with a genotype of Dd. If a dark-kin breeds with a Dd human, you have a 50% chance of a dark-kin from that pairing.
Now, this model is simplistic and doesn't match the story as in canon dark-kin seem to be more rare and more random than this would suggest. This would suggest that dark-kinism is controlled by multiple genes, like eye colour, to manifest.
Of course, once you add "because magic" as a reason, all of this goes out the window
