The Astral Empire: Heredity and Inheritance

(Disclaimer: the Abh are ultimately the work of Hiroyuki Morioka. This adaptation to the fourth edition of Dungeons and Dragons deviates significantly from the original.)

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In the original source material, the Abh reproduce almost entirely by technological means. Since the original Abh are gene-spliced human stock, the children of non-Abh nobles are gene-spliced in the same manner. There are several options for converting this to a fantasy milieu:

1. Directly. Abh are created using (expensive and/or secret) rituals instead of (expensive and/or secret) technological methods: the Abh were originally created as a slave race, and the means to create Abh from other races were preserved after the rebellion. Thus non-Abh nobles can and will have Abh children of their own, and access to these rituals — or to a facility that can perform them, probably in Lacmhacarh — is a right of every noble. This is the simplest conversion, leading to the fewest differences. (It is whimsically suggested that, in lieu of technological artificial wombs, the ritual's result be that Abh children are born from eggs.)
   
2. Unusually. Identify Abh with humaniform dragons. The child of a Abh and anything else is invariably an Abh. This is not terribly different from the above, socially speaking, but may appeal more to some.
3. Mundanely. To preserve the Abh supermajority of nobles, nobility is not hereditary for those not of the Abh race. Thus, landed non-Abh nobility will not in general be able to designate their own biological children as heirs of their demesne; such demesnes may often fall back into the Imperial Demesne, and such nobles may have many Knights as hangers-on. (This is significantly different from the source material, however: notably, Ghintec — a non-Abh, ennobled when his father was — is an unlikely character here, and the Baron of Faibdach is outright impossible.)
4. Complicatedly. Identify Abh with elves and 'commoners' with humans; half-elves are legally (although possibly not socially) equivalent to full Abh. Consider Abh-ness to be a Mendelian trait A/B (AA = full Abh, AB = half-Abh, BB = human), except possibly that half-Abh breed true. (Full Abh would likely be privileged — it's likely that the royal gens consists solely of full Abh. Ghintec's backstory is possible here, but any Lamhirh/Ghintec relationship is unlikely.)
   

In any case, a noble with a demesne is expected to designate another noble as their heir, who may or may not be their biological or adopted child; if a landed noble dies without a designated heir, the demesne formally becomes part of the Imperial Demesne, to be granted at Imperial whim.