Help me understand DNA testing

[Rob]

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.

[Alan H]

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).

An excellent point and absolutely correct, thanks for mentioning it.

[wvpiper]

An excellent point and absolutely correct, thanks for mentioning it.

Which was alluded to in this story posted two pages back
http://www.cbsnews.com/stories/2007/...n3334427.shtml

"And that's the rub. This business of genetic genealogy is fraught with limitations. For one thing, it can only provide information about a tiny fraction of our ancestry. Because we get half our DNA from our mothers and half from our fathers, almost all of our DNA gets shuffled and remixed every generation, making it impossible to trace what comes from whom. There are just two bits of DNA that remain pure - the "Y" chromosome, which passes directly from father to son, and something called mitochondrial DNA, which passes unchanged from mother to child.

Hank Greely, a law professor at Stanford University, has studied this new field. He worries that people don't realize just how many ancestors they actually have.

"Eight generations ago both you and I had 256 great-great-great-great-great-great grandparents," Greely points out. "It doubles every generation. So you've got two parents. You have four grandparents. You have eight great grandparents. Sixteen great-great grandparents. And it adds up fast. It adds up so fast in fact that if you go back 20 generations you've got over a million grandparents."

1,048,576 to be exact. And in each generation, DNA testing can provide information about only two of them. "

[ForresterModern]

It adds up so fast in fact that if you go back 20 generations you've got over a million grandparents."

1,048,576 to be exact. And in each generation, DNA testing can provide information about only two of them. "

Actually for the population it adds up to about 1.5. Men can find out about two of their ancestors (paternal y-DNA and Maternal m-DNA) while women can only find out about one (maternal m-DNA). Since roughly half the population is of either sex it averages to 1.5. Nit-picky, I know, but even more telling of the limitations of DNA testing.