The characters discuss at various points across both timelines the idea of diluting the Deryni bloodline with human blood. Whether or not this is merely an interpertation of the parties involved is unclear, but there does seem to be some evidence that the power level of one who is less than full breed is weaker than a full blooded Deryni, at least in the later books (in publishing terms).
All in all, at least for this campaign, the genetics of the Deryni will be outlined here to give some understanding as to what happens across the generations. This is idle speculation, more to explain why I set up the rules for the campaign the way I did than anything else, for two reasons. One, Katherine Kurtz herself is not clear on the issue (with her fans at least; she may know for sure how she wants it to work) and two, the people of Gwenydd have no idea of genetics themselves, except in imprecise terms of the early medival people.
The Deryniness Gene: This is a dominant gene for "Deryni power activation." The recessive gene marks one as "human." This represents the fact that having one parent who is Deryni usually produces Deryni children. This could be sex-linked to the female, but that wouldn't explain Duncan's Deryni son.
The Power Cluster: This is a collection of genes, some recessive and some dominant, that add up to determine an individual's power level and to some extent the powers one is capable of learning. Having a strong power cluster but not having a Deryni-on gene represents a human who is a good candidate for assumption of powers, but isn't able to learn to access that power on his own, not will he develop powers without help. This also represents the possibility of the dilution of power through the dilution of bloodline, in that with each human in a person's ancestry there is the greater chance to have more