Iku's mother was born from a Swallow Maiden (I think, the grandmother doesnt seem to have powers and shes married to a normal guy) so the powers might be hereditary but in a recessive gene, if that applies.
So the logic is that he sends out the powerless maidens out, produce offsprings, wait for a few generations until inbreeding stops becoming an issue, then marry back in anyone that is applicable. Any descendants he decides to not marry become village members and more swallow maidens, and he's immortal so he's got time to spare. On paper the village has a pretty diverse gene pool.
Though, realistically speaking, I have absolutely no idea what would happen if you have a child with your great-great-granddaughter.
TL;DR: If human genetics work anything like they do in animal husbandry, then the clan head's choices on how he manages his people don't make sense. Then again, this is typical in most settings, fantasy or otherwise.
Longform Version:
If a trait is linked to a recessive genes, that just mean an individual needs two copies of that gene to express the trait. Hones in goats are recessive, which is why if you been a horned goat with a horned goat, you get another goat with horns. Meanwhile, polled goats (no horns) are based on a dominant gene needing only one copy to have no horns. A goat with a single copy of each trait (dominant polled and recessive horned) will be polled.
More information here for those curious.
If this special power is a recessive trait, it needs two copies of the gene to express. No child of a Swallow Maiden should have more than one copy at
most.
If this special power is a dominant trait, then everyone in the village would have at least one copy. There wouldn't be Swallow Maidens to begin with.
So either something much more complex is happening genetically (wherein multiple different dominant or recessive genes have to be passed for the power to express) or this is your typical pseudo-science fantasy understanding where bloodline is all that matters. ;w;
In the case of the former, it would be
highly unlikely for Swallow Maidens to have children with powers (assuming people outside the village carry none of the required genes to "complete" the ones the mother is missing, unless like the polled goats, a single dominant trait is all it takes to suppress the expression of powers).
With the latter, having power or not is luck of the draw, and saying that the most powerful must marry to pass on the strongest power is often utter rubbish.
Third option? Power = inbreeding, in which case, Swallow Maidens make even
less sense.
This Swallow Maiden practice is to bring in fresh genetics or
outcross, but this only brings in new "vigor" if these new children marry other powerful individuals. Repeat that over several generations of marrying less powerful individuals to more powerful individuals and one would eventually get children with strong powers. Perhaps this is what happens to the clan head's children?
If you made it to the end, thanks for reading this rather haphazard and likely only half true explanation on my understanding of genetics in animal husbandry (because I have no idea when it comes to people.)
May your attention span increase tenfold.