Note: This text bases on the info given in the article Dragons - an inside look. If you aren't familiar with terms like DNA, I suggest you better read that one first, especially as the whole theme is a bit complex and may not be quite easy to understand anyhow.
Only recently some interesting facts about Dragon's DNA in comparison to other's DNA have been developed.
When looking at the DNA of a not yet hatched, not yet bonded dragon, a new kind of molecule inside them was developed. No one yet knows how it is built exactly, that's why it got the name Strange-(S-)molecule. This molecule appears at different places within the DNA. First there is a long string of it at the beginning (or ending - depending on how you see it of course) of it. This string consists only out of S-molecules and has about the length of the DNA of a human (a rather interesting fact). But there are also several S-molecules spred all over the rest of the DNA, at these places there are only a few pairs (sometimes only one) of them together, between them are the normal, familiar molecules. Altogether, each dragon in this stage had the exactly the same DNA.
When looking at the DNA of a dragon that has bonded, it was discovered, that all these S-molecules were gone, having changed into the common molecules (really a strange behaviour). It was further found out, that the long string of S-molecules was replaced by a string of DNA that exactly matches the DNA of the human who bonded with the dragon while the short strings/pairs of S-molecule spred all over the rest of the DNA seem to determine things like colour, gender, size etc. of the dragon (at least there have been found similarities in these parts between dragons of the same colour, gender etc.).
It seems therefore as if the dragon's DNA is determined just by the time it bonds with a human, no sooner. At this time, the dragon finds out the DNA of the human it will bond with and adjusts one part of its own DNA into the same as the human's. That way they don't only share their minds, but also the DNA (but this could explain how it is possible to share each other's minds, as they are same in one part). At this time - and no sooner - it's also determined which colour, gender etc. the dragon will have, also by chaning the DNA into the way it should be.
written byHeike