Forensic Science Question...please help?

A simple way to define this, ifs your dna is like a bar code, its individual to ones self.. so when leaving say sweat at a crime scene, they can lift the dna from the scene and match it like fingerprints.. in parenting cases, their will be bars or that are the same and esp in boys where the y chromosone is passed down to the son..

however their is always some problems and in very rare cases you will find chimeras where an individual has two sets of dna.. and this has caused problems in identifying parents if they get the wrong set of dna.. this either can be something like if they have had a transplant or blood transplant..