Here's the kind of thing that I think about in my spare time....
In many fantasy settings that I have encountered (and particularly in Dungeons and Dragons campaign settings), dragons, as well as powerful otherworldly creatures such as angels and demons, have the capacity to crossbreed and produce viable/fertile offspring with nearly any kind of corporeal creature. From this, certain results in the world at large in these settings might be expected.
This trait of omnifertility, combined with the clear/massive advantages to survival (and, in the case of descendants of beings from the celestial realms, appearance/attractiveness) conferred by their supernatural bloodline over typical members of their mundane species (advantages which might persist in being passed down to some dergee over several generations of crossing only with the mundane species of the original cross) would mean that semi-dragons, semi-demons and semi-celestials (et cetera) would have an evolutionary advantage over their fully mundane (or rather more fully mundane, as I will explain presently) conspecifics. As a result, their draconic or extraplanar genes might be expected to be spread far in the genepool, while at the same time being diluted (or at least, as a rule of thumb, not significantly increased) down to a certain limit depending on the overall size of the genepool and the frequency of supernatural crossbreedings, with the end result being that eventually most (or more likely, all) living corporeal creatures' genomes, regardless of species (more or less; although it could be (most likely validly) argued that sentient species might be expected to contain a significantly higher percentage of celestial genes on average than subsentient species), would contain some percentage of both draconic and otherworldly DNA, without the overall appearance of those creatures being significantly altered (EXCEPT in the cases of small isolated populations with small gene pools, and/or populations situated in close proximity to a dragon's lair or extraplanar portal and in which crossbreedings could thus be expected to occur more frequently)
Additionally, inheritence, in whole or in part, of the panfecundity trait enabling these crosses in the first place could go a long way towards explaining such anomolies as half-elves and half-orcs