Neither. All four sentences have article mistakes. They should be
- In this paper they study the genetic structure of the human body. (the required here)
- They (have) examined over 100 patients...
As for the perfect construction with have, probably that's not necessary.
Scientific reports generally use specific tenses and constructions for different parts of an experimental paper. For instance, if the experimenters built a piece of equipment that is still available, they might use a perfect construction, but, if the piece of equipment is not still available, they'd use simple past instead.
Your best course of action is therefore to use whatever tenses and constructions the paper uses in your own report. If they wrote "We examined", you write "They examined"; if they wrote "We have examined", you write "They have examined".
Don't try to second-guess the author, or the author's grammar, unless you're the editor of the journal it's being published in.