Comparing parent child common matches for continuity

+3 votes

Hi all

I did an interesting exercise a couple of months ago. 

The context is that from what I can determine, regardless of the site, I have no one closer than a fourth cousin who has also taken a test

However, I was finding some very decent sized single segment matches beyond the MIRC, if an outer range was set. 

So in FTDNA and gedmatch, I ran my father's matches against mine to find our common ones. Using basic v-lookup in Excel, I as able to compare them based on the top segment. 

I found that about 96 percent of my our common matches were either carried forward verbatim (I used my phased kit in gedmatch to avoid any of my mom's) or to smaller extent depreciated by less than 10% (for example a 30 cm would be more than 27 cm.

I was wondering if anyone had done a similar type test.  I think we get a sense from the literature that there is more "half lifing" of them than there really is. 

In my case, it seems to be an all or nothing - and the matches suggested that some of these families just seem to have a way of latching on longer than others, sometimes very long in situations where only one familial source is the basis for the family tree intersection.

Interesting exercise - wonder if anyone else has any thoughts on it from their own personal experience or from an expert point of view.   


in The Tree House by Lloyd de Vere Hunt G2G6 Mach 4 (46.0k points)
retagged by Lloyd de Vere Hunt
Interesting question. I don't have an answer for you, but you'll have a higher likelihood of someone coming up with an answer if you add the DNA tag as well.
Thanks - I have done so now
Please excuse my ignorance; what is "MIRC"?

1 Answer

+3 votes
I have quite a few of these as well, where a segment (30 cM or smaller) passes fully intact from my dad to me. I have the same effect on my mom's side. ( I don't have many close cousins on either side that have tested so I'm playing around with fourth cousin results too) So Gedmatch doesn't really change the generations to an estimated common ancestor from since it's based on the amount of DNA.

I could see where these segments sort of stick together, but you only get half of the segments, instead of cutting the DNA in half. (For instance- my mom's admixture comes up with 19% UK where I only inheirited 2%, so I must have not inherited those segments, but I tended to inheirit her others instead)

Without getting to technical, DNA is long codes made up of C T G and A, grouped by three's, the DNA is read to make diifferent substances. So a code could be GCG AAA CAC TGT ATA. It would make sense that there was some way to insure "AAA CAC" sticks together so it doesn't make nonsense code and make an error that makes a baby non-viable.

These segments and the fact that I keep getting matches estimated to be 4th cousins who actually are 10th cousins (with no closer brickwalls that would make 4th cousin possible) have got me wondering about what I think of as "sticky segments". (Some of those matches I can partially accept there may be even more distant pedigree collapse we can't see, when they were Mayflower ancestors or something where it may've had a smaller breeding pool that are boosting results)

 It makes me wonder about our accepted ideas of how our matches should share our DNA. I think a lot of these ideas are based more off of statistics and the chance that DNA doesn't get passed down. So you have 64 fourth great grandparents who all had 23 pairs of chromosomes. that's 46 chromosomes times 64=2944 Chromosomes of info. So I can see where it's rare to have had the same info passed down two seperate paths, and I think that's how many charts are written. The probability instead of the outlier chance.

In my mind, it's more like getting copies of half a book collection (solid pieces, also copy's because you inheirtiting it doesn't bar other descendants from getting it) than a copy of half a gallon of water (a liquid mixture that can dilute and mix).  Ok, I admit I need to work on that metaphor a bit, but a book collection kinda works- each person has a set bookshelf size and can only hold so many.
by Allison Schaub G2G6 Mach 1 (16.7k points)
Thanks for your affirmation - more people need to look into this - because I am not hearing enough success stories and so many people getting frustrated and ticked off because they don't fit the perceived mean average or the MIRC - I have only three matches where I have hit that range compared to many others - all older - and actually more valuable because they carry more generations provided they go into a particular nook and cranny of the tree that is unique.

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