Thursday, August 16, 2012

Autosomal Testing of Multiple Family Members



We all know that autosomal DNA is inherited is rather random way. Although each parent contributes essentially half of a child’s DNA, it is hard to predict how much of that half comes from each grandparent. We are all still learning how best to understand the test results.  

Testing the autosomal DNA of multiple family members can have several advantages. One of the more obvious is that test results for a paternal or maternal relative can greatly reduce the part of your family tree you need to search when a match from an unplaced relative is made. Since I do not have the option of testing my parents I have used a paternal first cousin and a maternal first cousin – once removed as surrogates.  I made a small experiment to see how helpful the autosomes of my cousins might be. I also compared the test results of my son and daughter with those of my 57 projected “distant relatives” who were predicted to be my 3rd or 4th cousins by FTDNA. The table below shows my preliminary findings:

Family Finder "Distant Relatives" 
Predicted 3rd or 4th Cousins
"Distant Relatives"
Me
1st Cousin
(father’s side)
1st Cousin -- 1 removed
(mother’s side)
Daughter
Son
Total
56
38
47
43
47
My closest
"Distant" matches
No other match

My matches that also matched one of these close relatives
1-10
0
2
1
6
2
11-20
2
0
2
5
5
21-30
1
3
1
5
3
31-40
4
2
0
5
2
41-50
8
0
1
0
1
51-57
2
0
0
1
3
Total
17
7
5
22
16

As you see 7 of my 57 “distant relative” matches were shared by my paternal first cousin. Also 5 of my 57 matches were shared with my maternal first cousin – once removed. As I had expected none of these 12 matches overlap. So it appears that a match with one or the other of these two cousins should narrow my search for our common ancestry to one side of my family.

Other findings were not as predictable. Twenty-two of my “distant relatives” were shared with my daughter and 16 with my son. Of those 5 of their matches overlapped and they had 28 unique matches between them.

My children share one of their 5 common matches with each of my two cousins. In addition, each child independently shares one separate match with each of my cousins. Actually, my son shares two with my first cousin – once removed that are not shared with his sister.

Of my 57 matches the 14 strongest matches are shared with at least one offspring or cousin and 23 of the strongest 25 are so shared. Only 11 of the next 25 are so shared. Four of the weakest 7 matches are shared. It is possible that some of the 19 unshared (with close family members) actually would show up as “remote matches.” I may extend my little analysis to see if this is true. If I do I’ll add the results to the blog at that time.

Also of possible interest is to explore further the extent to which full siblings share matches with distant and remote relatives.  

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