Y-DNA is passed from a father to his sons. Women do not have Y-DNA because they do not have the Y Chromosome. A father normally passes his Y-DNA to each son exactly as he received it. Measuring 111 markers from the father’s Y-DNA will yield the same results as each of his sons.
However, occasionally there will be a change (called mutation) in the result of one marker for one son. In this case, a father who had five sons would have four whose Y-DNA would look exactly like his, and one son whose Y-DNA would look exactly the same except for that marker. From that time forward, the sons of the man with the changed Y-DNA will now have exactly the same as their father, which means they will also have one marker that differs from their grandfather, uncles, and cousins. Over a period of several generations, it is possible to map out which person belongs to which specific line of the family by charting where the mutations match.
Mutations don’t occur in every generation, nor can we predict when they will occur, or to which person. Some families may have Y-DNA that seldom changes while others will have gone through more changes. At the same time, testing companies have developed averages that help predict how closely two people may be related to each other. Generally speaking, two people who match on more than 102 of 111 markers are related within a relatively recent set of generations. FTDNA, for example, predicts that two people who match on 109 of 111 markers have an 80% chance of being related within 4 generations. As mutations increase, the likely number of generations separating two people increase. However, there is no precise answer to the number of generations, which means that genealogical research is critical in helping determine exactly where the intersection for the common ancestor occurs.