As others have said, it can be/is a genetic condition. I still have all but four of my milk teeth, one of my cousins has all of her milk teeth (she's blonde, by the by, whilst my hair has natural auburn streaks in it), and my daughter (another blonde) only lost half her milk teeth. I'm 40, my cousin is 34 (and was written up in a dentistry text book as an example of the condition), my daughter is 19. My son (11 and with auburn streaked hair, too) is also looking to hang onto his milk teeth as he's only lost 3 teeth... although that might change.
I'm surprised that your DD was X-rayed, though. I was under the impression that dentists don't like to expose children to jaw X-rays until they're 16 - for any reason. My DD wasn't X-rayed until she was 16, despite this being a known genetic possibility, and my DS hasn't had one done at all to determine if he's going the same way (although, as he's closer to 12 than he is to 11, it's probable that he'll hang onto his milk teeth...)
There is a thought, though, OP, that the lack of adult teeth to follow the milk teeth is because the adult teeth have already been cut (ie, your DD may not have had milk teeth after all, but cut the adult teeth first). Certainly that's what my family dentist believes to have happened with mine and my DD's teeth. We also don't have 'wisdom teeth'. At all. My dentist explained that it means we're probably part of the evolution chain... because humans no longer need the 'wisdom' molars at all, so we're slowly starting - as a species - to not have them (my cousin and I were also born without tonsils!)
How old was your DD when she cut her first tooth? I was 13 months old before I cut my first tooth, and my grandmother was apparently looking to see if any dentist would make dentures for a toddler... There may be a correlation, though.
To cut a long tale short, OP, I'd not worry. Your DD won't be left without teeth, one way or another. 