Step 3.1 Maternal or Paternal ancestor?

Can you tell if the match is on your maternal or paternal side?

There are several ways to determine this:

1. If you are a male, and you match someone on the X chromosome, the common ancestor has to be on your mother’s side. (For females, a match on the X does not narrow it down to either maternal or paternal side). This is a particularly useful technique if you have only tested yourself and no other family member (e.g. adoptees). X-matches are detailed further in Step 3.2.
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2. If you have tested another family member (e.g. parent, aunt, cousin) then anyone who matches both of you has to share a common ancestor with you on that particular side of the family.
   
   For example, if you have tested a family member on your mother’s side (e.g. mother, mother’s sister, maternal 1^st cousin, etc), and you both match the same match, then the common ancestor is on your mother’s side. Similarly if you have tested a family member on your father’s side, a match with both you and your paternal relative confirms a match on the paternal side.
   
   The reverse is also true where parental matches are concerned – if a match matches you but not your mother, then the common ancestor is on your father’s side. Similarly, if a match matches you, but not your father, then the common ancestor is on your mother’s side.

Have other members of your family been tested? If so, check to see if they also match your match.

Below is a diagram of my family and the members who have been tested (highlighted in yellow) – me (MJG), my siblings (KJG & ED), my father (MHG), my maternal aunt (JH), and my father’s 2^nd cousin (ALH). 

• If I have a match who also matches my maternal aunt (JH), then I know that the common ancestor is on my mother’s side, and I can immediately rule out all the ancestors on my father's side. 
• If I have a match who also matches my Dad (MHG), then I know that the common ancestor is on my father’s side, and I can immediately rule out all the ancestors on my mother's side. 
• If in addition they match my Dad’s 2^nd cousin (ALH), then I know that the shared DNA has to have passed through my 2x great grandparents JG & AG and the common ancestor is  somewhere in the tree above these two individuals, perhaps one of  their parents, grandparents, and so on...

A Worked Example 

FTDNA’s FF results indicate a match between MB and 3 of the 6 members of my family that have been tested. MB matches ED, KJG & JH. ED & KJG are my sibs (sister and brother), JH is my maternal aunt. This indicates that the match is on my mother's side, and not my father's. This is confirmed by the fact that MB does not match MHG (my Dad) or ALH (my Dad’s 2^nd cousin via his father’s father’s father).

These matches are indicated in the diagram above by the green arrows. The yellow squares are those members of my family who have been tested. The fact that MB does not match me (MJG) is pure chance due to the random recombination of genetic material as it is passed down from our common ancestor. To learn more about the science behind the transmission of autosomal DNA have a look at this brief video from SMGF.

MB has persuaded some of her cousins to test and this will tell us whether the match is on her maternal or paternal side. I have asked some of my cousins to test and this will tell us whether the match is on JH’s maternal or paternal side. This will substantially reduce the number of candidates for our Common Ancestor.

Links & Further Information

1. ISOGG Wiki page on autosomal DNA
2. DNA testing for genealogy - getting started Part 3: autosomal DNA by CeCe Moore, Geni.com blog, 1 August 2012
3. Autosomal DNA by Angela J. Cone
4. Ask a geneticist by Dr. Erin Cline Davis, a science writer at 23andMe
5. The atDNA gamble: playing the odds by Judy G. Russell
6. FamilyTreeDNA's FAQ page relating to autosomal DNA testing