[chiefalchemist]

Would it be safe to say it's not the percentage of difference that matters, but the role of DNA that's different?

The point being, not all DNA is equal so to speak. That a couple of changes can have massive impact?

[haihaibye]

Yes, a single base change can give you a disease, cause intellectual disability or make you never be born.. There are mutations we never see in adults, as they are embryonically lethal.

Generally, you can tell what matters by seeing if it's been under selection - ie the frequency of that version in the population changes more than randomly.