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I have done the Autosomal DNA test through Ancestry.com and uploaded it gedmatch.com.

In Ancestry.com I matched several people that would be 1st or 2nd cousins. I also matched with them on gedmatch.com.

The problem is that I do not have any of their family names in my tree. My parents and grandparents are all deceased. My dad was an only child.

I can trace and connect with cousins on my mother's side and only my grandmother on my dad's side. My DNA matches all have a last name in common that I do not have.

I am beginning to think that my dad is my dad, but my grandpa is not my biological grandpa. So... I am not sure where to go from here in my family history.

My question is if you match on your DNA with other's that are 1st or 2nd generations then how could that show up if we don't share a common relative? If you are 1 generation, does that mean cousin or could it be sibling?

Could this Autosomal DNA test be wrong in its relationships?

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I have heard of rare occasions where kits were mixed up. That being said, did you find any other matches that were correct? If so, I would assume the test was correct.

1 generation in gedmatch would indicate a very close relationship. My mother shows up as 1 in mine, and I'm guessing siblings would be the same (I'm unaware of how 1st cousins might show up in there, but it wouldn't hurt to ask Gedmatch how it might show up). My second cousin shows up as 2.4 generations. I don't have other relatives in Ancestry, but in Family Tree DNA, my mother shows up as parent/child, my great aunt shows up as 1st Cousin, Aunt/ Uncle, Niece/ Nephew, and my second cousin shows up as 1st Cousin - 2nd Cousin. In my experience, my 3x great grandfather had a first marriage and child that we only learned about a few years back, so it's quite possible you have an extra line there that you are only discovering now.

If you think your test is correct, you can contact your close matches that shared the common name, and find out where their ancestors were located. That might place your ancestors in the same time and place as theirs, and you can go from there.

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Core Answer on are Autosomal DNA tests ever wrong on the relationships? Yes, as relationships are just estimates.. BUT at the distance you are talking about not really.. Once you get beyond 4th cousin it does becomes a bit more difficult and below 15 shared cMs the estimates have some false positives as they may have been accumulated from multiple relationships...

ISOGG has a great chart here.

Definitely Check out the answers of : Generational Loss of DNA testing.

As you have described it with the basic detail (cM levels would be helpful), assume that your grandmother had your father with another man and your grandfather adopted him as his own OR very few if not any members of his family with close relationships have tested yet.

I would approach this a couple different ways:

Do not get hung up on last names; last names change for lots of different reasons and focus on their commonality.

Find out what the people you do match to, that do not match to your mom's or grandmothers side have in common. Usually a location is a good place to start and then also work on family names on both paternal and maternal sides. Use GEDMatch and ftDNA.com's tools for "not in common with your mom's side" to decrease the noise.

Find brothers, sisters, cousins of your grandfather your dads side and see which one of their descendants have tested or talk them into it; usually offering to pay for the test works well for me.

If you haven't also transferred your Ancestry results to FamilyTreeDNA.com I would recommend it as they also have another matching pool to tap into and have some other tools as well.

I would recommend a Y-DNA test of NO LESS than Y-67 to help determine your true paternal line.

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