If my 1st cousin and I each have our DNA tested what will the compared results prove?

Question

If my 1st cousin and I each have our DNA tested with Family Tree DNA Family Finder, what will the compared results "prove"?

Will I automatically be able to tell if future matches with others are through my paternal or maternal lines based on whether or not they match with my 1st cousin as well?

If it matters, the cousin is a male and his mother is my father's sister.  We can be reasonably sure there are no close relations between his father and my mother, nor between my mother and father.

Thank you for helping to educate me, I'm sure others will learn as well.

Have a great day!

 

Answer 1

If you share the expected amount of autosomal DNA for your relationship then it will confirm you both share the same grand parents.  If you don't share other (unknown) ancestry with your first cousin and you both match another person then you can assume the shared ancestry is through your father.  If your cousin does not match a person you do then it is uncertain if the shared ancestor is through your mother or father.

It is usually easier to learn more by using Y-DNA matching with distant direct paternal line cousins or using mtDNA matching with distant direct maternal line cousins.

It is also usually easy to confirm ancestry (and more ancestry) with autosomal DNA matching between you and your 2nd or 3rd cousins (because you don't have the complication of having to use triangulation).

Comments

Thank you sir!

Answer 2

If your autosomal DNA match is strongly consistent with a first-cousin relationship (see https://isogg.org/wiki/Autosomal_DNA_statistics for statistics to compare with), you can be confident that you really are biological first cousins.

And after that, as Peter says, when you compare your data and your cousin's data with new DNA relatives, it can help indicate which side of the family the new relative comes from (assuming you and your cousin are related on only one side of your family). If both you and your first cousin have good matches with a new DNA relative, it's a good indication that the new relative is on that side of the family.

Because this cousin is a male and his mother is your father's sister, you will not share yDNA, mitochondrial DNA, or X-DNA (except possibly by coincidence). X-DNA is part of the autosomal DNA test. If you should find a DNA relative who shares X-DNA with him, that cousin is assumed to be on his mother's side (your father's side of the family). If you find a DNA relative who shares X-DNA with you, that relative is on your mother's side.

But none of this matching is "automatic" -- you need to look at the data and the genealogy. WikiTree DNA tools are designed to help with the data analysis.

Comments

If you find an unknown DNA relative who has a fairly close DNA relationship with you, but not with your first cousin on your father's side, that's a strong indication that you the unknown DNA relative is on your mother's side.

For example, consider the case of my DNA cousin at 23andMe who is an adoptee, and who matches with me and my sibling at a 3rd cousin level (just under 1% of our DNA, on 6 to 7 segments). If my sibling and I have a good match with  my first cousin and this mystery DNA cousin has no match with my first cousin, we can conclude that this mystery cousin's parent was on the other side of my family.

Lack of a match becomes a less solid source of evidence as the quality of the DNA relationship with a mystery relative becomes increasingly distant.

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Thank you very much Ellen!