Y-chromosome DNA testing is performed to determine whether two or more males are biologically related through their paternal line (father's side of the family). 
 
A Y-chromosome test is used to verify a biological family relationship when a male’s alleged father is unavailable for a DNA Paternity Test. A son inherits his Y chromosome from his father, and the chromosome does not change from father to son. (The Y Chromosome is said to mutate once every 10,000 years) Because of this, a male's Y chromosome can be tested and compared to check whether the individuals are biologically related through the paternal family line. If the males' in question are related, their Y Chromosome will be identical.  Please note, while a Y-chromosome test can demonstrate that 2 males are paternally related, it cannot determine the nature of the relationship such as whether they are father or son, grandfather or grandson, uncle or nephew etc.