Isolation through Elimination Possibility?

[Tspoone]

My (half)brother and I share a mother. He has gone through the DNA testing to verify his father's lineage. I am female, have no male relatives from my father's side. Father is deceased and I am the last of my line. Of course my simple little brain reasoned that if I were to have a DNA test done, and we excluded any markers my brother and I have in common - then the remaining markers would have to have originated on my paternal side...right? Okay - feel free to explain my faulty logic to me now 'cause I know it can't be that simple. (Just be kind and use small words with not too many syllables.)

Thanks!

[Lklundin]

My (half)brother and I share a mother. (...) if I were to have a DNA test done, and we excluded any markers my brother and I have in common - then the remaining markers would have to have originated on my paternal side...right?

Not always, but in some cases yes.

You got half your atDNA from your mother, and your brother got half his atDNA from your mother.

But the two halves you got cannot be assumed to be identical - and are most likely not.

So while you are right that any (sufficiently large) segments you share with your brother must come from your shared maternal side, you are (both) likely to have other atDNA segments from your mother as well.

So if you take the test, then any (significant) matches shared with your brother would be maternal, but each of you can have additional maternal matches as well.

But you can have a match with a 3rd person and be sure that the matching segment is paternal, in the case where these conditions are met:
1) you have a significant segment matching a 3rd person, and
2) on the same location you have a match with your brother, and
3) your brother does not have a match with the 3rd person on that location.

Such a matching segment would be explained by you matching your brother on that half of the chromosome pair that you got from your mother, and you thus matching the 3rd person on that half of the chromosome pair that you got from your father.

The same would apply to such a combination of matches occurring on the X-chromosome.

I am female, have no male relatives from my father's side.

If you and a female relative on your fathers side tested, then you could triangulate on matches shared with her.

[Tspoone]

That makes a lot of sense. Thanks so much for the clarification!

[tomcat]

My (half)brother and I share a mother. .. Of course my simple little brain reasoned that if I were to have a DNA test done, and we excluded any markers my brother and I have in common - then the remaining markers would have to have originated on my paternal side...right? ...

I recommend you both take the 23&Me test as 23&Me does both ancestry estimation and cousin matching on the X chromosome and the X chromosome can be directly attributed to one or another parental line.

Your half-brother's single X came from your shared mother. One of your two X's came from your shared mother. Your other X came from your father, who is your testing target.

In comparing your two X's to your half-brother single X, there is a range of possibility -
1) Your half-brother's X may be wholly identical to one of your X's
2) Your half-brother X may be wholly different than either of your X's.
3) Your half-brothers X may be somewhat similar and somewhat dissimilar to one of your X's.

(The odds favor 3).

In the first instance (1 - wholly identical), you will have isolated your father's X or half his genomic bequest to you on the sex chromosomes (males are XY). Hence, you will know with certainty that any matches on X that you and your half-brother do not share are attributable to your father's "identity."

In the second instance (2 - wholly different), you will have doubled the amount of available potential information about your mother as you will have both her X's without having tested her. Hence, all X matches of your half-brother and, nominally, half of your X matches should conform to your mother's "identity" and any that don't conform can be provisionally attributed to your father's "identity."

In the third instance (3 - most likely) you will be somewhere in-between 1) but with less information with which to isolate your father's X, and 2) but with less information about your mother. All your half-brother's X matches will relate your mother's lines and all the X matches you share with him relate to your mother's lines but only "some" of the X matches you do not share with your half-brother will be maternal. Nominally, half your own X matches will be paternal.

[mkdexter]

we excluded any markers my brother and I have in common - then the remaining markers would have to have originated on my paternal side...right?

No, not right. Sorry.

You will have to first use the not-in-common filter between you and your brother's kit, and then choose each person and compare where they share with you, to where your brother shares with you in the chromosome browser.

Any time you see they match in the places you share with your brother you can determine if they are paternal or maternal by whether they match both of you or just you. Any time you see they match in a place that you and your brother do not match, you will not be able to figure out if they are paternal or maternal to you.

None of this works very well with "Speculative" matches, just "Distant" or "Close" matches.

I've enclosed an example. If I go to one of two half sibs pages and then compare people that are in-common-with them both, I get matches in the places they share segments. If I go to one of the two half sibs pages and then compare people that are not-in-common with them both, I have to use the chromosome browser and choose the half sib, and the 3rd party and compare them. If they match in a shared sib area, but they don't share the match, that match (3rd party) is paternal. If they match in a non-shared sib area I will not know if that match is paternal or maternal to either of them unless I find another relative to compare with.

[Tspoone]

Thank you so very much for the explanation! I appreciate your taking the time to help me understand my options! This all offers an amazing amount of information possibilities for old geezers like me.