All humans have a pair of sex chromosomes that determine the sex of the person. The Y chromosome is determining the human carrying it to be male.
While females have two chromosomes of the same kind ( XX ) males have two different sex chromosomes ( YX ). The Y chromosome can only be passed on from father to son, a DNA analysis can therefore show a paternal relationship between two or more persons if a matching Y chromosome can be found.
This is valid for the complete paternal line, thus applies not only for a paternity but also for a relationship between uncle, nephew, grandfather and grand sons.In cases where a paternity test cannot be performed a Y chromose test can be very useful to determining biological relationships between male test participants. Males who share the same male ancestor will possess the same Y chromosome.