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I have a 107 cM match with Eli. His mother is Lily and Lily's brother is Henry. Henry's son is Henry Jr and his grandson is Kevin. I have a 235 cM match with Kevin. Kevin is one generation younger than Eli. I don't understand why there is a larger cM for Kevin over Eli. (I used fake names.)

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I am going to hypothesize that:

  • Lily and her brother Henry inherited 235cM from their parents.
  • Eli may have only inherited 107cM (of the original 235cM) from his mother Lily. He did not get the remainder by chance.
  • Henry Jr may have inherited all 235cM from his father Henry.
  • Kevin may have inherited all 235cM from his father Henry Jr.

By the above hypothesis, it is possible that Kevin, even being a generation younger than Eli could nevertheless share more DNA with you than Eli does.

You could test my hypothesis if Lily, Henry and Henry Jr were to test. You would not expect all the numbers to be right but you would expect children to always share less cM (or the same) with you than their parents.

  • Understanding cMs is new to me. Hopefully your hypothesis will be an example I can learn from. Testing of others is not likely. From my mother, brothers, and cousins the cM number seemed to make sense, so this was confusing me. – ekcan Jan 22 at 1:48
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Whenever you look at a specific relationship in DNA terms, there is a range of cM associated with it. The cM range for any given relationship will overlap with, usually, multiple other possible relationships.

We all start with 6800 cM. If you compare yourself to a test of yourself, or your identical twin, it will be more or less 6800 cm. You and your child will have a match of around 3400 cM. After that, it can get dicey.

Your child has approximately half her/his DNA from you and half from her/his father. But that half that is from you...how much is from your mom and how much from your dad? The average will be 25% each. Meaning everything divides neatly in half. But the reality is always a range. There's no way to predict which side of any given DNA strand will be the one passed down to a child.

Eli and Henry Jr. are first cousins and they share a single set of grandparents. In theory, they should each have 25% of their DNA from each of those shared grandparents. This means around 1700 cM inherited from any given grandparent (numbers are rounded). In reality, the range of cM from each grandparent is 1156-2311 cM. Add to that the fact that each grandchild will inherit different DNA segments from each grandparent, not just different amounts of total cM.

Eli's first cousin once removed, Kevin, may have, as PolyGeo points out, nearly all of the DNA his dad had from one or more of his paternal grandparents. Or he may have inherited very little.

Both Eli and Kevin will be within the cM ranges for the relationship they have with you. Use this information to narrow down what relationship you have to this family. Because of the disparate numbers, you have an opportunity to narrow down the possibilities more than most people do.

There are also other explanations.

You might match Kevin through both his mother and his father. This is not uncommon in general but, if you all belong to an endogamous ethnic group (any group that tends to marry people of that group, even if they are not known family, for example, Jews, Hawaiians, Mennonites, Colonial Americans, etc), then it's more common to match over multiple lines than just one line.

It is also possible that Eli's and Kevin's family has some secrets, but that is not the conclusion I would jump to. Everything you describe is perfectly normal and easy to explain.

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There are not many pairs of relationships where these cM levels would fit in the known ranges. While there are several cousin relationships (some fairly distant) which ranges would include Eli's 107 cM, there are not very many that would allow a more distant relationship at Kevin's level. One possibility might be Eli - 2C (46-515 cM) and Kevin - 2C1R (0-316 cM). I'm not suggesting that's the actual relationship, but you should probably be looking for something like that.

As previously noted, matches with other cousins on both sides of their families will provide additional clues. One additional data point that would be useful to know is the match level between Eli and Kevin, which might either suggest or rule out some of the zebras.

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