Y chromosome (Y-DNA) testing
A man's paternal ancestry can be traced using the DNA on his Y chromosome (Y-DNA). This is particularly useful because the Y chromosome, like many European surnames, passes from father to son, and can be used to help study surnames. Women who wish to determine their paternal ancestry can ask their father, brother, paternal uncle, paternal grandfather, or a cousin who shares the same paternal lineage to take a test for them (i.e. any male family member who has the same surname as her father).
What gets tested
Y-DNA testing involves looking at segments of DNA on the Y chromosome (found only in males) where sequences of nucleotides repeat, known as short tandem repeats (STRs). These segments are considered "junk" DNA. The segments which are examined are referred to as genetic markers and are designated by a DYS number (DNA Y-chromosome Segment number). These STRs may also indicate a likely haplogroup for the Y chromosome, though this can only be confirmed by specifically testing for that haplogroups' single nucleotide polymorphisms (SNPs).
Understanding test results
A Y-DNA test will look at between 10 and 43 markers on the Y chromosome. The results will tell how many repeats the test subject had at any given marker; the variations of repeats are known as alleles. For example, at DYS455, the results will show 8, 9, 10, 11 or 12 repeats (source). The test results are then compared to another person's results to determine the time frame in which the two people shared a most recent common ancestor (MRCA). If the two tests match on 37 markers, there is a 50% probability that the MRCA was less than 5 generations ago and a 90% probability that the MRCA was less than 17 generations ago.