Cheers, Killer Clowns, for that insight.
In one of my WIP I was thinking of a system of magical genetics that functions like a magically-affected recesseive gene with extremely predictable results when breeding amongst themselves or with normal humans or those who carry the recessive gene (who are easily identifiable as they have predictable traits:
Magical immortal + Magical immortal —> 100% chance of magical immortal offspring as all offspring carry two of the recessive allele.
Magical immortal + normal human —> 100% chance of "immortal" offspring with no magical ability at all
(carrying the recessive allele confers immortality automatically)
Immortal + Immortal —> 25% chance of Magical immortal (2 recessive alleles), 50% chance of Immortal (1 recessive allele), 25% normal human (no recessive allele at all).
Immortal + normal human —> 50/50 chance of immortal or normal human offspring, depending on whether the child carries the allele.
Your post makes me realise that I need to consider what effect this would have on their breeding choices, given the predictability and detectability of the results.
In the culture to which these magical/immortal people are indigenous, there is a "balance" in that no immortal person (fully magical or not) is permitted to rule - presumably back in the mists of time they discovered the problem of having an unchanging ruler in a changing world - but the advantages of their magical ability and immortality are not trivial, so I dare say there would be those deliberately selecting mates based on their genes.
For contrast, there's another, non-hereditary, form of immortality that can be gained by those who see its advantages for those in power.