Second cousins shared DNA.

[kiwilong]

Hello.
I struggle with understanding DNA and would welcome advice regarding the following please.
I am male and have a 2nd female cousin and we are descended from a common great grandfather.
However we each have an individual great grandmother, a blood relative.
How would this situation affect our shared DNA and relationship, ie, second cousins or half second cousins?
I think we may have an amount of DNA, subject to normal variations, as if our great grandmothers were the same person, ie, a normal family relationship but I can't figure it out.
I would really appreciate some advice.
kiwilong

 

[At home in NZ]

I struggle with understanding DNA

I would too which is why I don't do it. However, I wonder if you've read Peter's Masterclass which you can find here
I hope it helps.

 

[Pauline]

Two of my great grandfathers had children with more than one woman, and I refer to those descended from the same great grandfather but not from the same great grandmother as half (or half blood) 2nd cousins. The chart in Peter's masterclass (as linked to by AHNZ) indicates that the average DNA shared with a half 2C is 117, and the range 9 - 397cM.

I have at least 4 half 2nd cousins in my matches and share between 97 and 140cm with them. My sister shares 322cM with one of those and only 67cM with another. So it is, as you say, very variable.

 

[peter]

I struggle with understanding DNA

All you really need to understand is that children inherit half of each of their parents' DNA, and that it's essentially random which bits they get - everything else flows from that.

Unfortunately there are numerous bloggers (and even some LostCousins members) who make it more complicated than it really needs to be. You can safely ignore anything that talks about chromosomes or triangulation, and as for the abbreviation, cM, by all means think of it as centimetres - it isn't, but it might as well be.

 

[kiwilong]

Thankyou all.
My question concerned a match of 195cM across10 segments and because I was comparing it to a chart by SEMANTIC CONSULTING which states the range to be 19-197 and 1 to 9 segments the 195 cM seemed to be on the high side, the reason for my concern.
I have a copy of Peter's master class and had noted the figures of 9-397 but had decided to use the more conservative table. Anyway, I'm happy now especially being able to compare with Pauline's results.
Can I take this opportunity to thank Peter for the news letter, which I always enjoy, and also the masterclass, both of which have assisted me greatly over the years.
Regards.
Geoff Long.

 

[peter]

Although on average half 2nd cousins will only share half as much DNA as full 2nd cousins, the randomness of inheritance means that it's quite possible for a pair of half cousins to share more DNA than a pair of full cousins. Best to stick to trusted sources to avoid confusion.

 

[kiwilong]

Thank you Peter.
Briefly, my maternal grandmother was illegitimate, born 1887, and her mother was early 20s and born disabled. So I am looking for a half second cousin, unknown to my family, and DNA has provided such a person, shared DNA 195 across 10 segments whose direct ancestors, her great grandfather in particular, 3 years older than my great grandmother, lived not far from my great grandmother. More importantly I have a second DNA match, same surname as the other match and my current match. All 3 families are Manchester people back several generations.
So I think my current match and me share the same great grandfather.
But I was not really sure about the amount of shared DNA for half second cousins, hence my post.
Having the second match, more distant, and the surnames of my current matches parents is a stroke of luck.
I know the answer is not 100% but on the balance of probabilities!!
Geoff.

 

[peter]

The fact that you have three matches isn't necessarily significant - it depends on what the relationship is between them.

But let's go back to where this discussion started - you stated that you had a half 2nd cousin, but reading what you've said subsequently it would appear that you were only hypothesising that the person you have been matched with is a half 2nd cousin.

In other words, you have a close match with someone who isn't on your tree, and you are hoping that this is a clue to the identity of your unknown great-grandfather. Hopefully you're right, but there are other possible explanations which - on the facts presented - you are unlikely to be able to rule out at the current time.

If you have any DNA evidence that your match is related to your through your maternal grandmother that would be an enormous help - hopefully your mother had one or more siblings, and one of them (or their descendants) can test.

 

[Tim]

Have you been able to triangulate on these matches Geoff? There is a great tool that shows you all the possible relationships which I find really useful.
If you have the number of cMs that you share with these other matches, I's be happy to do a WATO tree for you. WATO is What Are The Odds.

 

[Pauline]

my maternal grandmother was illegitimate, born 1887, and her mother was early 20s and born disabled. So I am looking for a half second cousin

A match who offers clues towards the identity of your unknown great grandfather may not be a half 2nd cousin. You will only find a match with a half 2nd cousin if your great grandfather had other children, and if one of those other children has a living descendant who has done a DNA test, and is in the same generation as you.

You may be more likely to find a match who is related to your great grandfather, but with whom you share ancestry a generation or two further back - so a 3rd or 4th cousin, perhaps.

 

[kiwilong]

Thank you all again.
When I started this topic I knew how limited my DNA knowledge was but now realize how abysmally limited it actually is.
Regarding Peter's comment, which is correct as i had assumed that the match was a second cousin, but i had already thought that a first cousin or closer relative with a match to my putative second cousin would be good. All my grandmother's children are dead, there were 7 cousins including me but only 4 of us are still around, 2 in America haven't tested but the daughter of the remaining one has and is my biggest match, 294 cM/18. Two messages have been ignored to date. Her site is managed by her husband apparently.
I will try again and explain in more detail what I am looking for.
I hadn't seen the "possible relationship" chart before Tim. The WATO tree sounds interesting and the two matches are 195/10 and 9/1 if that would work, thanks. No reply to my message to the 9/1 match so far.
Pauline's comment is noted as well but would it be fair to say that I would be very lucky to prove absolutely who the gentleman was and perhaps my being contacted by a descendant of one of his other children, 5 by his wife, would be required.
Geoff.

 

[Pauline]

There is a great tool that shows you all the possible relationships which I find really useful.

I agree it can be a useful as an alternative to looking at the options in the chart, as although the information is the same, it's been extracted for you and presented with probabilities.

However, not everyone fully understands probabilities so it has the potential to be misleading - just because something is more likely to be the case doesn't mean it actually is! I am sure I will not be the only one who can quote examples from my DNA matches where the least likely (or even an omitted) relationship is the documented one.

One of the benefits of my (full) sister having also tested is that I can readily see from the very different amounts of DNA we share with the same match, just how wide these variations can be.

 

[Bob Spiers]

All you really need to understand is that children inherit half of each of their parents' DNA, and that it's essentially random which bits they get - everything else flows from that.

Unfortunately there are numerous bloggers (and even some LostCousins members) who make it more complicated than it really needs to be. You can safely ignore anything that talks about chromosomes or triangulation, and as for the abbreviation, cM, by all means think of it as centimetres - it isn't, but it might as well be.

I thought your answer Peter deserved more than just a tick or a thumbs up; positively right on the ball and couldn't have put it better. I especially like cM thought of as centimetres having recently tried to explain about the 'real' cM to a cousin. Talk about the blind leading the blind.

I still think your example using the distribution of playing cards as set out in a past Newsletter (or was it the Forum) simplified the random inheritance of parents' DNA explained things far better than the usual 'mumbo jumbo' DNA bloggers seem to adore.

 

[kiwilong]

Hi all.
I have sent a nicely worded message via Ancestry to my first cousin's daughter asking if she would mind checking her results for the same two cousins and/or surname.
The husband still seems to be active on Ancestry.
Here's hoping!!
Geoff.

 

[Tim]

One of the benefits of my (full) sister having also tested is that I can readily see from the very different amounts of DNA we share with the same match, just how wide these variations can be.

Yes, I have also tested one of my sisters and it does show the differences. But still a valuable exercise to do as she had matches that I didn't have and vice versa.

 

[peter]

Hi all.
I have sent a nicely worded message via Ancestry to my first cousin's daughter asking if she would mind checking her results for the same two cousins and/or surname.

Since the relationships are all so close you should be able to find this out by looking at your shared matches.

 

[peter]

Yes, I have also tested one of my sisters and it does show the differences. But still a valuable exercise to do as she had matches that I didn't have and vice versa.

Yes, and not just matches you don't have, but shared matches you don't have, ie where your sister shares more than 20cM with a genetic cousin, but you don't.

 

[kiwilong]

Just wanted to thank you all for your input, much appreciated.
Regards.
Geoff.