I did some research on paternity tests a while back and it's very possible that a paternity DNA test wouldn't pick up anything odd unless we're talking about some seriously weird issues with Clark's DNA (in which case it would be unlikely he could be Christopher's father.)
Paternity tests are very specific and compare specific sequences. No one will be trying to look at Clark's genome for anomalies. All that will be looked at are those specific areas and computers do the work.
Assuming Clark is Chris's father, chances are very good that a DNA test (assuming Clark's DNA can be used in a test in the first place - but again, if Clark is Chris's father, his DNA has to behave 'normally') wouldn't throw up any red flags.
