Comment: As rare as

(See in situ)

In reply to comment: "They have been able to (see in situ)

As rare as

But up til now, they haven't even produced a very sick chicken, let alone a dinosaur.

They've found a that can cause chicken embryos to start growing alligator-like teeth, although the mutation is fatal. Perhaps that qualifies as getting to the sick chicken stage?

http://news.sciencemag.org/2006/02/mutant-chickens-grow-teeth
http://www.scientificamerican.com/article.cfm?id=mutant-chic...
http://www.paleogenetics.com/articles/archosaurianteethchick...