DNA Randomness: Siblings

If you have been researching genetic genealogy for any length of time and I guess more so if you have also taken courses or attended webinars,  you have heard about the randomness of DNA as it pertains to autosomal DNA inheritance.  While we each get 50% of our DNA from our father and 50% from our mother,  the 50% that we get from each parent is not the same 50% when comparing to other siblings(unless you are comparing Identical Twins).  I have 4 full siblings, only one of which has tested.   Even with just one having tested I can see that is true in comparing our matches and the amounts she and I match them.  Our Mother has tested also, so I have phased kits created at GEDmatch using her results in comparison with mine and also phased kits comparing her results to my sister's results.  These are very useful.

Ancestry has a phase filter which you can use if one or both parents have also tested at AncestryDNA.  Since I only have one parent that was able to test, that tool lets me see a listing of matches who are also a match to my Mother.  Keep in mind the match range is similar to Shared Matches tool in that it only includes 4th cousin and closer matches---not Distant Cousins.  It would be nice if they could do a list of 4C or closer who DO NOT MATCH when there is only one parent who has tested.

The chart below compares my match list with that of my full sister.  I did not include our Mother's results for our maternal matches nor the matches of my daughter.  I may do that in a future post  Those covered are matches who Ancestry DNA estimated were in 3rd cousin range for at least me OR my sister.

The RED border is around those who were in my list at 3rd Cousin or above.
The VIOLET border is around those who were in my sister's list at 3rd Cousin level.who were ranked as 4th cousins by Ancestry in my list.

Average Difference 65 cM
Total-M(Marie)
Total-B(Becca)

Notes:  Both of our parents have Colonial American Ancestry.  Pretty much all of our ancestors were in the US before 1800.
Our Distant Cousin matches Start on:
Page 41 of 876 (Becca)---- 30 matches which are 3rd Cousin or closer
Page 38 of 908 (Marie)-----26 matches which are 3rd Cousin or closer
Marie:  1873 4th cousin or closer matches
Becca:  2007 4th cousin or closer matches

GEDmatch comparison:  Marie and Becca
Largest segment = 145.6 cM

Total of segments > 7 cM = 2,507.6 cM
51 matching segments
Estimated number of generations to MRCA = 1.3
Six of my matches that were estimated 3rd cousin were in the 4th cousin range for my sister.  Eight of my sister's matches that were estimated 3rd cousin were in the 4th cousin range for me. There was one match who was on the low end of a 3rd cousin match for my sister who was in my Distant Cousin match.



Ancestry's Cousin Categories
From the Hyperlinked "What does this mean?" in the Amount of Shared DNA info box





A bit confusing I guess as there is a gap between categories. I did check for ours that fell
in the gap(195 cM) and it was noted as a 3rd cousin.

Visuals of DNA randomness

Segments Mapped to the Grandparents

Looking at a Segment map where 3 of the 4 Grandparents have tested and there is a phased kit with the DNA of the untested Grandparent.  It is easy to see that while we can be genealogically kin to someone the lines get whittled down at random.  

There are several instances more so on the Paternal side where the entire chromosome copy is from one grandparent.   4(PGF) 3(PGM) 0(MGF) 2(MGM)  

Granted there are still DNA segments which passed thru each of the grandparents, when entire chromosomes are passed, especially the larger chromosomes, the other lines which didn't "make the cut"  are no longer represented on that particular chromosome.

(Image below is a segment map generated from my database using Genomate Pro.)

PGF(Paternal Grandfather) PGM(Paternal Grandmother)

MGF(Maternal Grandfather) MGM(Maternal Grandmother)

Paternal Phased Kit Comparisons

Running a One to One comparison of the Paternal Phased kit of my sister(full sibling) against my Paternal Phased kit 

Ignore the lower part of the line(Blue & Gray) as this is Phased kit comparisons and only contains Paternal DNA  

Green=Match

Red=No Match

I did not include the X in the comparison because as full siblings and females we have an identical Paternal X chromosome. (Image below from comparison tools at GEDmatch.com)

Working AncestryDNA circles and a few observations

My sister and I have both tested at AncestryDNA.  When I am working on a segment and it is pointing in the direction of a specific ancestor, I always check my AncestryDNA circles to see if I have a Circle for that particular ancestor or ancestor couple.  I work the circles.  What do I mean by work?  I have a spreadsheet template I use.  I begin by adding the Ancestor's name at the top.  The spreadsheet has columns for:

• Ancestry User Name
• Match or Match to Circle
• Estimated cMs/segments
• Group(Child thru which they descend)
• GEDmatch #(known and added as they upload)

I add the info for each circle I work and this helps me to track not only who matches me but the descendant pathways.  Also and probably the most important thing for me is that I can see at a glance which tester would be the most valuable to have their info at GEDmatch.   I generally write those who Ancestry estimates match me on multiple segments or at amounts greater than 15 cMs.  I ask them to consider uploading to GEDmatch if they are not already there and if they have uploaded, how to identify them among my many matches.  I want to focus on these larger matches for a number of reasons. The circle membership gives me a pretty good idea of where they could match me. It is easy to get overwhelmed by all the matches but I feel if you focus on the larger ones, those will help in identifying others.   I also like to look at the smaller matches and see how their estimated segment matches are in comparison with my sister's results.  If they match her at amounts over 15cMs I also message them. 

Stay focused on the larger matches when trying to get folks to GEDmatch.  However do not forget the randomness of DNA.  Siblings can vary greatly in what matches they have.

My sister and I are a great example of that.

Since Mom tested I was able to make phased kits(thanks to GEDmatch tools) which show the matches that each of us got from Dad(who passed before we began DNA testing.)  I have compared my Paternal Phased Kit to my sister, Becca's Paternal Phased Kit and also compared each of our Maternal Phased Kits.   Below are the results

Comparing Paternal Phased for Marie and Paternal Phased for Becca(Full siblings)

(Chr 1-22, X not included)

Largest segment = 134.6 cM

Total of segments > 7 cM = 1,526.2 cM

33 matching segments

Estimated number of generations to MRCA = 1.6

Comparing Maternal Phased for Marie and Maternal Phased for Becca(Full siblings)

(Chr 1-22, X not included)

Largest segment = 145.6 cM

Total of segments > 7 cM = 2,513.8 cM

50 matching segments

Estimated number of generations to MRCA = 1.3

So you can see that on our Paternal lines we appear to have gotten a greater variety, while Mom seems to have given us more of the same.  

Table below shows another example of the randomness. The amounts shown are those estimated by Ancestry and they tend to be slightly higher at GEDmatch.

Comparing segment matches we have to
members of a Paternal Ancestor Circle.