I think Ruth is the way he is because of the time he spent in the egg before Jaxom helped him to break the shell. I'm not the scientifically minded type, so I couldn't tell you the technical side of it. I might need to reread the book again to better understand the question, or give a better answer, I'm afraid, although there are some here who might be able to answer it better than I can.
Ruth was the runt of the eggs and no one thought the egg was going to hatch. It probably wouldn't have if Jaxom hadn't intervened. His "traits" go along with being the runt, he is smaller than other dragons (more the size of the first dragons) and his hide appears white, although really has all the colors of the dragons within it, and there has never been any other dragon of this color. His unique abilities do play a key role in the stories of Pern, but I don't know where you are in the series, so I don't want to give anything away for you.
Ah, I forgot about Ruth's unusual abilities. I wonder if that would have been a direct result of his protracted time in the egg, or if it's something he would have been born with anyway. Chances are it's because of the time spent in the egg, but you never know. After all, in Skies of Pern, ordinary dragons were revealed to have some very different abilities, too. Sorry for going off-topic a bit.
Ruth was my favourite when I read the series a couple of years ago ( Although I haven't read the last few).
As for an explanation, I'm afraid I have nothing new to add to what's already been said here.
Ruth.... physically he is white with washed of the other dragon colors, as well as being nearly genderless. there is a large amount of speculation within the sotries if the dragon makes the rider or if the rider makes the dragon. Ruth started in an undersized egg, and would not have lived if not for Jaxom, but the pair of them not fitting into either world, holder or dragonrider made their presence felt in both. as far as abilities, RUth has the same set of abilities all dragons have, but his deals with the one thing that is constantly dismissed in dragons. memory. he has one, now it COULD be because of size or genetic variation (fire-lizards have memory if not sentient level intelligence) always knows his current position in time and space.
Ruth is my favorite dragon, because he has a genuine personality unlike most others. I think it has to do with his better memory, you can't have personality without memory. However, reading about Ruth is excruciating because there's also Jaxom to deal with. Jaxom's just about the ultimate Gary Stu and I can't stand him.
In the beginning the dragons were engineered to meet a specific threat and size was of huge importance in facing that threat. The dragons were able to last longer in flight if larger and burn more firestone etc.
However evolution has some cul de sacs where genetic mutations, that ultimately prove unhelpful to the survival of the species as a whole, crop up only to lead nowhere. Possibly Ruth is such a variation and only time will tell if the extra abilities he has are of lasting ie. evolutionary value to the dragons of Pern.
Alternatively, he could have been created as yet another answer to the question of what dragons will do when the skies of Pern are free of thread. Possibly future generations of dragons will develop in a more Ruth-like way. This could be more useful to Pern and easier to support than the traditional dragon.
I can remember Aivas discussing Ruth and asking if "sports of the Species" were kept. Ruth was a unique situation across the board. only thing I can think of close is a stem cell. Ruth appears to have ALL the genetic material for Dragons without any specialization. the size is a minus, no desire to propogate his genes is the other.
he was referred to as a sport several times in All The Weyrs Of Pern and Jaxom touched Ruth's egg before he hatched causing a special link between him and Ruth maby that is why ruth has a better memory
That might be it, Great Hunter. As the only white, there's no real comparison to the other dragons and their skills to know whether it was due to pre-impression influence or the fact that he's a sport - or some other factor (Ruth's time in the egg, before Jaxom, for example). Unfortunately, it seems that Ruth will be the only white dragon in Pern's history, so those skills of his definitely make him one of a kind.
I think Ruth can be considered the next step in dragon evolution, maybe even as a slightly failed experiment by Mother Pern to do just that. Ruth could be seen as a dragon born with a birth defect because of his size and color (and genderlessness), but then there are his abilities to consider. Ruth can represent loss and gain coexisting in one entity. His personality and uniqueness also makes him a very significant character in the storyline, unlike most of the dragons in the other books. They are more like minor characters though they are linked to those that play a major part. I hope that isn't confusing...
It would also be interesting to see if any Weyr records were uncovered, mentioning the occasions when a white (or a sport dragon of any kind) was born, and also to see if more like Ruth will be born in the future. Given that the dragons will eventually never need to fight Thread any more, the ones with bigger brains and better talents for telekinesis would be more useful. Just a shame that Ruth wasn't interested in mating - I wonder what kind of draconic bloodline would arise?
Pardon me if I'm going to ruffle any feathers here, but I always felt that situation with Jaxom and Ruth was a kind of oblique nod to the difficult issue of being homosexual in a generally straight society. It seemed to me to be about choosing to be different, or being born different, and conservative or traditionally-minded people having problems with that difference, but eventually accepting it because there's no other option.
Jaxom never felt comfortable as a nascent Lord Holder, even before Impressing Ruth, and afterwards he can't belong to the community of Lord Holders because he's a dragonrider, and can't belong to the dragonriders because he's a Lord Holder. Ruth is also clearly out of step with the other dragons by virtue of his size, colour and odd abilities. The two have a very close and intimate relationship, closer than most dragons and riders, no doubt because both feel out of place within their own communities. And Ruth is a girl's name, traditionally. Yet here it's given to a male dragon, and a male dragon which refuses to breed. Which means, of course, that Jaxom too would normally remain celibate.
I can't remember if he does, actually. Doesn't he develop a relationship later? It's been years since I read The White Dragon.
Anyway, just a passing thought on the possible authorial intentions behind that storyline. Could be wildly wrong. Though it's a theory that could be developed ...
well, homosexuality is at least accepted in the Weyrs. I wouldn't say Ruth is a feminine name, though. come to think of it, I can't remember any other dragon names with a u in it. that might be faulty memory though.
Jaxom and Ruth... Okay Jaxom, born ceasarian after his mother died. Ruth also born alive through outside assistance when he couldn't crack his own egg. Shell too thick, and a membrane too thick to push through as well. as was made reference to in othe books, stillborn eggs were taken between and left there (by the weyrleader) so it was not unheard of, someone coming along and cracking open an egg, that was unheard of.
The society was described as rigid, but even then there was some fluidity. the only real line was that a dragonrider must stay in the Weyrs. this is the one rule that was broken for Jaxom and Ruth, mainly because the majority of humans felt that Ruth's days were numbered from the outset. Once that proved false the decision to not allow the pair into the fighting wings was implemented. This was the main point in TWD that was stressed. Jaxom was Lord Holder in name, since Lytol was Warder, Ruth was a dragon by birth and species, but was not any of the "traditional" colors for the species. as such he also did not mate, some felt it was because he would never mature to a specific gender. So Ruth is a dragon that defied all conventions. no Color to define him, no mating to define his gender, That taken away removed one of the times when a dragon's bestial urges took over a human nearly completely. He still ate messily, but hey can't have it all. Last was his memory, could speculate that since he was an anomolous creature having basically all the genetic code for the draconic species in him with no variation to seperate him into gold, brown, bronze, green, or blue, he might have kept the traits of dragons, but also have held some of the fire-lizard traits, with their racial memory etc... no actual information has come out of the stories, so its pretty hard to tell otherwise.
I will point out that on a number of occasions, the Here-and-Now memory touted to the dragons has been cast aside. with, say Mnementh, bringing up memories of things ten+ years in the past, his and F'lar's first solo flight, or Ramoth telling Lessa she will remember something til the end of her days. I dunno, Pern is still a great group to read, but rules bent or broken to tell a better yarn.