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My mother (age 96) shares 102.1 cM with a man (age 72). However, I do not share any cMs with him.

What could their relationship be and why don't I share any cMs with him?

  • How many segments make up those 102.1 cM? – lkessler Feb 17 at 2:28
  • There are 6 segments and the largest is 39.7. Thanks! – Mary Ann Feb 18 at 2:44
  • What site are you using, and what is the minimum cM length it shows? – Ellen Spertus Feb 24 at 4:01
  • I'm using FamilyHeritageDNA. – Mary Ann Mar 16 at 23:42
  • Correction: MyHeritageDNA and the minimum cM length is 8.0. Thanks! – Mary Ann Mar 16 at 23:45
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You don't share any cM with the man because you have a 50-50 chance of inheriting any given segment from either of your parents. While it's uncommon for a 100+ cM segment total to turn into 0 cM in one generation (because segments split up randomly so most children would get a piece of one or more), it can and does happen. Just the luck of the draw.

Going to the Shared cM Project, we want to look for a relationship that accommodates both your match with the man and your mother's match with him.

There are plenty of possibilities. I will only name a few. If your mom's largest segment with him is over 20 cM, it points to a closer relationship than if the largest is under 10 cM, say.

  • Second cousin (man + mom) and second cousin once removed (man + you).
  • Third cousin (man + mom) and third cousin once removed (man + you).
  • Fourth cousin (man + mom) and fourth cousin once removed (man + you).
  • Thanks for the information. The largest segment is 39.7 of 6 segments. – Mary Ann Feb 18 at 2:41
  • That's a nice large segment @MaryAnn. It doesn't guarantee a closer relationship but it makes it more likely. – Cyn Feb 18 at 3:08
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There is a 50% chance of you inheriting each segment your mother matches on. With your mother matching 6 segments, there is therefore a 1/64 chance that you don't match on any of the segments your mother matches on. That's a pretty low probability, but is possible.

That's assuming all your mother's matches are real. Any match below 15 cM might be a random match.

Try to investigate the people your mother shares in common with this man, and see if you also share with the in common with people. That might help you narrow down how this man is related to your mom and to you.

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