Originally Posted by Cheesegear
Only because this response is easy...
Using the Emperor's DNA requires putting a needle in the Emperor and using what comes out.
No, because Corax is Corax. Not the Emperor. That's like saying that your DNA is the same as your Dad's. That's why they turned out as Super-Raven Guard instead of Primarchs.
Why didn't Dad just give Corax his DNA? Well, because what happened later is exactly what the Emperor was afraid would happen (and he warned Corax too).
To be fair, Corax could have simply did his sneaky thing that he's so good at and picked up toe-nail clippings or skin from somewhere.
And, actually, since the Primarchs don't have a mother, their DNA should be very similar to the Emperor's. Unless there was some weird and unspeakable experiments with women going on, and that's why they're all so different.