I don't know exactly how familiar you are with genetic genealogy, so I guess I should start from the beginning. Men carry Y-DNA, and they pass it on to their sons, and their sons pass it on to their sons, and so on. This has been so since the first man. The Y-DNA mutates at a somewhat regular rate. The mutations are passed from a man along to his sons, and they pass that on to their sons, and so on. Over the long period of the existence of mankind, it comes down to us that the Y-DNA is passed is along down the male generations, along with their surname. Thus the Y-DNA follows the surname. So, theoretically, all of the men of a given direct male descendant lineage will share the same Y-DNA markers, within a few mutations. A man's Y-DNA can be tested. His Y-DNA test results can be matched against a database of other tested males, and the ones who match him will have shared a common male ancestor with that man. He can then contact that matching individual, who is really his relative, and they can then share and compare notes on their genealogy. The Y-DNA mutations that occur periodically can mark the dividing points on a lineage. When a male ancestor has a mutation occur on his Y-DNA, he will pass that on to his sons, but his brother's will not. In that case, one brother's descendants will have one set of results, and his brother's descendants will have the same results, but with that mutation. So, the purpose of any DNA project is to try to assist family researchers in determining which line of a given surname a man descends from. This process can be very very useful in avoiding dead end research. It can often help researchers to break down brick walls in their research. The Y-DNA test results will also reveal a man's haplogroup, which will tell him generally which part of the world his line comes from. To see how it works, here is a link to our Whisenhunt DNA Project:
In the case of the Bell sisters, I located an individual who was a direct female line descendant of one of the Bell sisters (Nancy), and that individual had been mtDNA tested, so we then knew the Bell sister's haplogroup. The mtDNA is passed from a mother to all of her children, but only her daughters pass it on to their children, and so on down the line.
The interesting thing about the Whisenhunts, is that their Y-DNA should all match to within a few markers, because they are thought to have originally been from one family in Switzerland. So far, they don't match, so, we need several more Whisenhunt males to participate in the project before we can determine what is really going on here.
I hope this helps. Email me directly, if you have any more questions.