Chromosome Browser question

[rachelleleclaire]

So.. I can't find the answer to this anywhere online.
My test and a known male third cousin both match one lady. but in the chromosome browser we dont seem to share any similar segments.
It's my understanding that if you share a similar or exact segment with someone it's a fairly good indication that it's been inherited from common ancestors - depending on size of segment I guess, and keeping in mind the two different pieces of each chromosome...
So how come Our "in common with" match doesn't line up??

[prairielad]

In common with has nothing to do with shared dna segments. The in common with tool only looks at the names in your match list and the names in matches match list looking for names that appear in both.

This is just a tool to suggest you and match may share a common ancestor with the fellow in common matches.

It is possible that an in common match shares one common ancestor with you while they share a different ancestor with the match you are running the in common with tool on.

[rachelleleclaire]

Really? Weird... we should actually both match this woman on the same line, it's a pretty specific connection we have (me and third cousin), there's not really a lot of chance it could be from a different line..
but... if we literally don't all 3 match on any segment does it mean that it would have to be different lines???

[larzus]

It could be that there are three different segments, all passed down from the one common ancestor. You received two, your cousin received one that you did and a different one. The other lady received the other one that you did and your cousin's different segment.

In this scenario you would match your cousin on a different segment to the one you match the lady on. She would match your cousin on the segment which you did not get. So no exact match but still the same ancestor and showing 'in common with'.

Usually you will have a Common Ancestor Couple rather than individual. If so, perhaps one segment comes from each partner in the couple?

Or the third option is of course that there is descent from more than one branch. Quite common amongst endogamous populations.

Others may be able to express it better than I can

[prairielad]

Attached is a diagram representing a hypothetical instance of two chromosome inheritance between 3rd cousins. This instance would show the three 3rd cousin as In Common With but who do not share the same segment between the three of them.

This is not saying in your instance this 3rd person is also a 3rd cousin, it is just to show how matching segments between three 3rd cousin can be from different ancestors from their four common lines.

Larger picture, so you may have to zoom and pan to read labels

[prairielad]

Here is an example where 3 people are In Common With each other, but the 3 people do not share the same ancestor .

[dna]

Here is an example where 3 people are In Common With each other, but the 3 people do not share the same ancestor.

Thank you!

That was very nice!

W.

[rachelleleclaire]

So matching segments are not really a good way to tell if 3 people share a common ancestor..?
Trying to find other matches that are on the same line as this known third cousin.

[LynCra]

Matching segments is a good indicator of sharing a common ancestor. The point that is being made above is that "in common with" does not necessarily mean that there are matching segments.

[prairielad]

So matching segments are not really a good way to tell if 3 people share a common ancestor..?
Trying to find other matches that are on the same line as this known third cousin.

As LynCra states matching segments indicate shared common ancestor. I was just giving you examples in regards to your intial post on In Common With and the three of you not having a matching segment which you all share

So.. I can't find the answer to this anywhere online.
My test and a known male third cousin both match one lady. but in the chromosome browser we dont seem to share any similar segments.It's my understanding that if you share a similar or exact segment with someone it's a fairly good indication that it's been inherited from common ancestors - depending on size of segment I guess, and keeping in mind the two different pieces of each chromosome...
So how come Our "in common with" match doesn't line up??

In post 5, I tried to show/explain why 3 people who are In Common With, who do share common ancestors, don't all share the same segment with each other in Chromosome Browser. Common ancestors DNA present in each, but each just inherited different segments. They are in common with because they are listed on each others matchlist

In post 6, I give another example of why 3 people who are In Common With don't all share the same segment with each other in the chromosome browser. In this instance the three of them do not share a common ancestor(s).

As mentioned, in regards to In Common With Tool, it is not based on shared DNA. This Tool only looks at the Names on match list being compared, looking for names that appear on both list being compared. It just suggests
these matches my share some common ancestors.

[rachelleleclaire]

I understand all that... what I am trying to do is figure out which other matches likely line up along the same family line as myself, my third cousin and the lady who matches the both of us by looking at the chromosome browser.
Theoretically... if any other matches shared a similar significant segment with any of the three of us... it's likely to be from that same family line... even if this fourth match only matches 1 or 2/3??
Does that make sense? Maybe I need a break from this DNA, haha.