Yours is an important question: Im going to try and shed light on this - its complicated but sometimes difficult to apply.
See:
http://www.apsu.edu/oconnort/3210/3210lect06.htm
"
Bloodstain patterns tell a lot about position and movement during the crime, who struck whom first, in what manner, and how many times. This destroys most alibi and self-defense arguments for crime, and at the very least, trips most suspects up in their explanation of what happened. Over the years, criminals have tried many ingenious ways to hide, clean up, and remove blood evidence, but it's an area where criminal justice technology has always stayed one step ahead of them."
In addition, Contact dna patterns can be helpful.
See:
http://www.ornl.gov/sci/techresources/Human_Genome/elsi/forensics.shtml
and
http://www.dna.gov/basics/
Also finger print patterns on objects. Finger print (HCl salts) can yield dna/mtdna as
well as raw prints ....
Any time you contact something you potentially leave dna, especially
through sweat and body fluids. If you strike something with force you always contact dna.
...............
There is nothing in Joran's account which says that Steph used
deadly force on him at any time, but only struck him on his temple
one time (so they will examine his temple area ... they already did
a body exanmination on him).
On the other hand we have wounds on Stephany which Joran's
account does not explain. Forensics will seek to establish how those
wounds occurred.
The FBI might be helpful in this -
I wonder how much real forensics was run early in the Holloway case - I doubt
there was much if anything, but I dont know. Ive never heard of anything...