nvFAQ article: Dryad Genetics: an alternative explanation
This explanation basically requires one major change to genetics. This
is that unlike the XX and XY system we all know and love a female is XY
and a male is YY - this occurs in other animals on Earth, so it would
work.
So, instead of being XY a Dryad is ZY (Z being the Dryad gene). One
characteristic of the Z-gene is that any YY foetus is spontaneously
aborted in the early stages. This solves the problem of no male Dryads.
Or male Dryadic offspring.
However it does leave the problem of no male heirs in the Borune family.
So postulate a Y' gene that produces some method for it not being aborted
(say it looks like a Z gene to the mother's immune system). This gene
will also require abortion of any ZY' foetuses, which would probably
happen by a similar mechanism to that in the case of a YY in "real"
genetics.
One member of the Borune family was given this gene by the Prophecy.
Thus they can have male line children and none of their /male/ children
will carry the Z and all of their female children will. Their success
rate for pregnancies would be half that of a human-human mating, but....
Now, Garion doesn't carry the Y' gene; so he can't sire a male offspring,
but the prophecy requires it - hence the business with the trees; they may
quieten down the Dryad's immune system for the required length of time or
something similar.
Incidentally, this does mean that all of Garion and Ce'Nedra's daughters
are Dryads...
Written by Senji and Vard