People doing genealogical research will have identified their most distant ancestor. This ancestor might be themselves, a father, a grandfather, a great grandfather, or someone further back in their family tree. The researchers may want to know whether or not that ancestor is related to other persons who often but not always have the same surname. In doing traditional genealogical research, the researchers try to obtain written records that will tell them if their ancestor is related to the other persons. However, such records may be difficult to locate and may not even exist. Since the Y-DNA markers do exist for male descendants of the two lines being investigated, the researchers can compare the Y-DNA markers of a male descendant from each line to determine if the two persons are related in some way. The use of Y-DNA can't give an exact relationship between two people, but it can tell if it is likely the two persons are related via a common ancestor. People who are likely related to each other will have most of their Y-DNA values the same. The fewer the values that are different, the more likely that the two persons have a common ancestor.
Let's consider an example of the use of Y-DNA to determine if two people might be related. This is a real-life example, but the names and locations have been changed to protect the privacy of the two persons, both of whom are still living.
John Leigh lives in Colorado. His great grandfather on his father's side immigrated from Wales to the United States. John has researched his great grandfather's Leigh line in Wales and has identified his earliest known Leigh ancestor. John had his DNA tested with the hope that someone in England will have Y-DNA that is a close match with his Y-DNA, thus giving him a general idea of where in England his earliest ancestor might have lived.
Michael Lee lives in Virginia. He has researched his Lee line back to his grandfather on his father's side, but he has not been able to extend his family tree past his grandfather. Michael assumes that the Lee ancestor who immigrated to the United States came from England, but Michael does not know who that ancestor was or when he came to the United States. Michael had his DNA tested with the hope that someone who has identified the Lee immigrant will have Y-DNA that matches with his Y-DNA.
After both men had completed their DNA testing, they were notified by the company doing the testing that they have a close match with each other and that it is likely the two men have an ancestor who is common to both of the men. The presence of a common ancestor is important because the common ancestor provides a link between the two lines, thus establishing a relationship between the lines. Michael was surprised at his close match with John, because he never suspected that his family tree might go into Wales. John explained to Michael that the common ancestor might have lived before John's branch went to Wales. If this is the case, Michael's family tree might not go into Wales, even though John's family tree does go into Wales.
As I mentioned at the beginning of this post, this example is a real-life example of how DNA testing gave new information that brought two distant relatives together and gave one of the relatives an understanding that his family tree might go into Wales. Now, because it is likely that John and Michael have a common ancestor, they can assist each other in further research into the identity and whereabouts of the Lee immigrant who came to the United States. To help Michael with his research, John has identified several places in his family tree in Wales where sidelines could have sprouted from his main line and later furnished the Lee immigrant.