Since mitochondrial DNA only is carried through the matrilineal line, how can we determine ancestry using paternal line?  The key is…..the Y-chromosome!
Humans have 23 pairs of chromosomes; each pair consists of one chromosome from the mother and one from the father.   While 22 of these chromosome pairs are identical in males and females (they are called autosomal chromosomes), the 23rd, the sex chromosome, differs. Females have two X-chromosomes, whereas males have one X-chromosome and one Y-chromosome.  The Y-chromosome’s SRY gene determines male gender, and is passed solely along the paternal line with very little variation from generation to generation.  Because of this, like mitochondrial DNA, the Y-chromosome is an extremely useful genealogical tool.