Phogfan86 requested a simple explanation of what DNA testing could tell him about his ancestry. However, most of what has been posted so far (although technically accurate) would most certainly go straight over the head of most 6 year-olds. I hope that the following is helpful.

There are two types of DNA that are used for ancestral testing. These are the non-recombining part of the Y-chromosome (NRY) and mitochondrial DNA (mtDNA).

NRY is passed down the direct male line unaltered apart from rare mutations. Therefore a man’s father will have the same NRY signature as the man himself, as also would his father’s father, his father and so on stretching back a thousand years or more. Testing for NRY will tell you nothing about your mother’s father, or his father, etc. Nor will it tell you anything about your father’s mother’s father, etc.

Similarly, mtDNA is passed down the direct female line unaltered apart from rare mutations. However, unlike NRY, it is also passed on by a mother to her sons, but the sons do not pass it on to their offspring. Therefore a man’s (or woman’s) mother (and her mother, etc.) will have the same mtDNA signature as the man or woman themselves. Testing for mtDNA will tell you nothing about your mother’s father’s mother, or her mother, etc.

This leaves an extremely large part of your ancestry about which neither NRY nor mtDNA can tell you anything at all. If we go back to the great great grandparents level (of which there would have been 16 in total), NRY and mtDNA tests would tell you about just 2 of them, i.e. your father’s father’s father’s father and your mother’s mother’s mother’s mother. This leaves 14 great great grandparents unaccounted for (genetically speaking).

Furthermore, DNA testing on its own cannot identify who the 2 direct line great great grandparents were. For this you will still need reliable family records, but once these two individuals have been identified, you can be sure that they will have the same NRY (direct male line) and mtDNA (direct female line) as you have. This only applies to males, as females do not have any NRY to test in the first place, so they can only trace one great great grandparent genetically, unless they can persuade a brother to be NRY tested.

Often a particular NRY signature is strongly linked to a particular surname if it is a patronymic surname, so it may be possible to tell if you are possibly linked to that branch of the family. I say ‘possibly’ as there are likely to be other branches with the same NRY signature.

DNA testing is probably most useful in determining likely ancient migrations of populations. For this reason, the field of study is referred to as population genetics and requires DNA samples to be taken from a significant sample of a current population. It can lead to some unexpected conclusions. For instance, in Victorian times it was assumed by historians that the ancient Britons were either exterminated, or driven out of what was to become England by the invading Anglo-Saxons. Population genetics studies have shown that this was far from being the case, as only a very small percentage of present day English people are descended from these 5th/6th century Germanic invaders, the vast majority being descendents of the ancient Britons.

I hope that this explanation of the capabilities and limitations of DNA testing has been useful.