nvFAQ article: Dryad Genetics

It's really very simple (long, but simple): The dryad invariably has one "human" X chromosome and one "dryad" X chromosome. When a dryad (female, of course) and a human male produce a female child, the male has donated an X chromosome, which still pairs with during mitosis and meoisis so that the child can grow and produce eggs (not so strange - the Y and X chromosomes of humans are extremely different and still pair in cell division, and in this case, it's the dryad X that varies slightly from the human X). Now, in human females, one X chromosome in _every_ cell of the body becomes inactivated after a few weeks of life. (It becomes super-condensed because a double-dose of genes would be too much - it is then called a Barr body). In dryad female offspring, the human X serves to determine sex of the child, and then is the one condensed in every cell of the dryad's body (in humans, it is a random choice in the cells, but it could _easily_ be forced in dryad cells, because the two X chromosomes are slightly different, so one can be recognized as the human one and then condensed). The simplest mechanism for this would be a protein or series of protein reactions derived from the dryad X to inactivate the human X.

Thus, a female child develops via the dryad X. Because half of her genome is human, dryads appear very much like humans. (The dryad half couldn't be much different anyway, because all chromosomes have to pair in cell division).

Males: Child receives human male Y and a dryad female X. The male Y chromosome codes for a gene product (TDF maybe - the testis (male) determining factor) that inactivates portions of the dryad X chromosome that deal with dryad physiology. This is entirely possible - on all chromosomes, areas of the chromosome are regulated separately and turned 'on' or 'off' at the genetic level by protein and DNA packing regulation. Thus, a protein from the Y, normally meant only for some human purpose (such as TDF, which develops a male into a male) could also bind to the dryad X or cause some sort of signal cascade that deactivates the specific dryad genes - remember that the dryad X is quite similar to the human X which can clearly be indicated by their stark similarity to humans, so all the normal physiological genes needed from the X chromosome remain active. So, there you go. Gene regulation and inactivation. Simple, real, human genetics that can be applied to the dryad system, which must be amazingly similar to ours if human males can actually manage to reproduce with them.

Re: "Half-dryad" and "full-dryad" nomenclature
Or, why there are no full dryads

All dryads are half-dryad because half their genome is from a human, in which case, the definition for "dryad" becomes "half-dryad," and the word "dryad" still is correct: Dryads are all half-dryads, but are still called dryads, because there are no full-dryads.

An analogy: the word hypercholesterolemia. This is a disease in which a person has one of two genes defective for a certain receptor that allows cholesterol to be taken out of the blood. That is to say, the person has one mutant copy of the gene and one wildtype (normal) copy.

Hypercholesterolemia breeds true (it is genetic, so obviously...), but the definition refers to a half-hypercholesterolemic, technically...

However, people with two copies are _not_ called hypercholesterolemic (ack, bad example! Typing that is annoying! ^_^), because there aren't any. Two copies of the mutant are lethal, so all "full-hyper'emics" are dead. Hence the word, like dryad, technically means a half-breed, but that's _all_ it means, so it is still the correct connotation. "Dryad" _means_ half-breed, and there are _no_ "full-dryads." So, essentially, when Polgara refers to Ce'Nedra is a half-dryad, it's still what we're all used to.

My explanation way above also explains how a female with only one dryad X can have the same phenotype of a theoretical full dryad.

Re: Children of dryads, especially males.

Without a deletion, half the male children of a dryad would have all their daughters (grand-daughters of the dryad) be dryads again; the gene causing dryad-ness in females would resurface after a generation. So, deletion of this gene by male children of dryads is likely possible, probably through an induced inversion mechanism, although it deoesn't really matter.

Actually, if a dryad X underwent auto-inversion in males across the centromere of the X chromosome... the dryad X chromosomes would break up. Thus, sons of dryads could only have male kids, and the dryad chromosome would be lost in that first generation.

Would anybody ever notice that particular orphans found near the Wood of the Dryads only have male children. Probably not, actually. Medical records in Tolnedra were probably sketchy.

The necessity for such a mechanism depends on whether a dryad is in any way recognizable from a normal human. Suppose the gene is not normally deleted, and a dryad-borne male is dumped into Tolnedra as an orphan (as per Aphrael's theory). He has three children in his lifetime, two of which are female, and thus both are dryad. _However,_ he doesn't know he's carrying the dryad genes on his X chromosome (which is given to all female children), and so nobody ever realizes they're dryad. _And_ if they're never bonded to a tree... then they don't have an extended lifetime (we know, despite the dryad/tree argument going on currently, that tree age is the reason the dryads live so long. Xantha states it). So, nobody ever realizes she's dryad, and nothing ever comes of it...

If this is the case, there's probably a lot of unclaimed dryads running around clueless in Tolnedra. ^_^. Personally, I prefer the deletion/inversion theory, because it's simpler, neater, and wraps everything up just fine. Occam's Razor, n' such.

Written by Rumor, repeated from the AFE FAQ.