Can 2 cousins determine through a dna test whether or not they have the same gra

Question:Let's say both cousins are related through their father's side, but neither have parents or grandparents still alive... so that rules out the possiblity of getting a dna sample from them. But if 2 cousins were to take a dna test themselves, is it possible that they could determine whether or not they had the same grandparents on their fathers' side?

Best Answer - Chosen by Asker: Let's say both cousins are related through their father's side, but neither have parents or grandparents still alive... so that rules out the possiblity of getting a dna sample from them. But if 2 cousins were to take a dna test themselves, is it possible that they could determine whether or not they had the same grandparents on their fathers' side?

If both cousins are male and related through their fathers to the grandfather, then their DNA will match because they have the same Y chromosome.

But if one cousin is male and the other female this won't work.

The male cousin will get his Y chromosome from his father/ grandfather and the X chromosome from his mother.

The female cousin can only get the X chromosome that her father got from his mother (the shared grandmother) and an X chromosome from her own mother.

Need to ask TedPack or Mind Bender on this one. They can usually set you straight. Sorry i can't help you.

By the fact that they are cousins means they share a common ancestor. Whether it be grandparent, great-grandparent, or even a g-g-g-g-g-g-grandparent. In order to be cousins, there MUST be a common ancestor somewhere on the family tree.

DNA will establish paternity. I don't know about grand-paternity. I'm pretty sure it would, and that it would cost more than the standard $149 12-point test to see if you have Native American markers.

It sounds like you suspect someone was adopted or someone's mom had a fling, or someone's dad has a checkered past. If two people have the same grandparents, they are first cousins. If they have just one common grandparent, they are half first cousins, although no one but a genealogist showing off would use that term.

Most people know who their grandparents were, but the company I used for DNA testing said 2% - 5% of the samples they get don't match like the donors thought they should, due to "secret adoption or infidelity".

This is the company that I used:
http://www.familytreedna.com/faqtip.html

You could write and ask them if they will test for it, and if so how much they would charge.

I'm assuming that the specification of "cousin" means that the THINK they are cousins, but aren't sure - if they were sure then why a DNA test?

DNA testing is not necessarily as straightforward as many believe. In this case you are testing 2 individuals believed to have a common ancestor. But you are NOT testing the "believed" common ancestor. This is an important distinction.

There are multiple types of DNA testing. But really the only thing ANY DNA test can show with what is called absolute certainty is that two people ARE NOT related. Thus if the 2 cousins took a DNA test, they can be told that they are NOT related - of course meaning they don't share the same grandparents.

But it can also show, to with a level of probablity, that there is a relationship between the two. But it is a probability. For example, if a DNA test is performed on a baby and it's mother immediately after birth, ignoring testing errors, sample contamination, etc., the test will still have only a 99.993% accuracy rate. That's pretty darn accurate, but not 100%.

But what a DNA test cannot do is determine a specific ancestor (unless the ancestor themself is part of the test). It can indicate, often to within high probability that there IS a common ancestor, just not who that common ancestor is (it could be one person's father who the second didn't realize was their father as well, it could be the grandfather, great grandfather, etc.)

There are also specific DNA tests that work on Paternal decendancies and Maternal decendancies. The problem with these is that there must be a common sex (male or female depending on the test) lineage back in time. In the case you presented, a Paternal DNA test (male lineage) would support the hypothesis since they are looking at grandparents (this would only apply to the grandFATHER) through their male parent. But again, even if there is a "match", it would come with a probability (maybe 95%, maybe 80%, maybe 98%) of a common male ancestor - but not who that ancestor was.

What you are doing is testing your hypothesis. And that is exactly what DNA testing is perfect for. You have information that makes these people believe that there is a relationship, thus the DNA testing could support (or disprove) this hypothesis. And because you have such a hypothesis, even an 80% probability would be sufficient to prove this for most people.

So in summary, yes, it can support or disprove the hypothesis of a common paternal grandparent, but not PROVE. It could be that you would have a common paternal great grandparent or parent. While in the legal world, the probabilities aren't often good enough for non-civil situations, in genealogy, they are often quite good enough. Especially when supported by other evidence.