DNA that is present on the Y chromosome of males is also found in the nucleus of the cell. This Y-DNA is inherited only by males from their fathers. The diagram below shows the inheritance pattern of the Y-DNA in males. Those represented by a circle are female and those represented by a square are male. All males with the same interior color share the same Y-DNA.

Due to the low mutation rate of the DNA on the Y chromosome, all males who share the same paternal lineage would be expected to have the same Y-STR profile, going back many generations. This can be very useful to determine if two living males have a father or fore-father in common, even if the father or fore-father is deceased.

The laboratory procedure for Y-STR analysis is very similar to that used in routine STR analysis. The only difference is that the various STR regions of the Y-chromosome are inherited from father to son as an entire unit called a haplotype.

There is some variation in the Y-STR haplotypes in the male population because there are instances of mutations where by the son’s Y-STR profile is slightly different from that of his father. This creates a potentially brand new haplotype that will be passed along to the male children of the son and further down that paternal line.

In a Y-STR test, if the Y-STR profiles between two individuals match, the matching haplotype is compared to a population database to determine the frequency of that haplotype in the male population of the race of the tested individuals. The final results are presented as the probability of shared paternal lineage.

If the Y-STR profiles between two individuals do not match, the two males are said to be excluded as having the same paternal lineage.