Why would my cousin matches that should also be mine are not?
I am managing my cousins kit for the first time. He is getting matches that I should share with him. The tree that I have built have those matches last names. Is it possible that he matches with some of our our possible shared matches and I do not?
Answers
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DNA varies even between siblings. We do not all receive the same DNA from our parents. Each person is unique.
Betseylee Browning
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I assume that these matches are not on your cousin's line that you do not share with him. I also assume that these matches are greater than 15 to 20 cM since below this level it becomes increasing more likely that matches are do to inaccuracies in determining segment data by the testing company.
Based on this assumptions, the reason you do not share some matches with him that you should is because you did not inherit the same segments that the match and your cousin did. To give you an example of this, I have tested my two siblings and me. I then looked at my top 2,000 matches at gedmatch person A. If I combine my match list with my first sibling's top 2,000 matches and eliminate the duplicate matches, I find that together we have 3,134 matches. That means that my first sibling has 1,134 matches that I do not. The reason is that we did not both inherit the same atDNA segments from our parents. If I now do the same with all three of us, I find we have 4,011 unique matches, which means that my second sibling has 877 matches that neither I nor my first sibling have.
Thus, the reason you do not have the same matches your cousin has on the same common side of your family is because you and your cousin have not inherited all of the same atDNA segments. I hope this makes sense. If we all inherited the same segments from our parents, grandparents, etc., then we would all look the same.
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If you are second cousins or closer to any individual, you should match. If you are more distantly related than that, you might match.
Some possible reasons that your cousin does not match:
- The individual matches to the parent where you do not have shared ancestry. (My cousins from America match on my paternal side. They would not match my maternal cousins from Europe.)
- The distance is greater than second cousins.
- One of the people involved does not have the expected ancestry. They might have an unexpected paternal event. (Daddy wasn't who they expected him to be.)
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@Aura Erickson - There is one other factor that @Carolyn Webber didn't mention. You get half of a parent's DNA but it is randomly determined. Two siblings can each get half but their half will be different. I have two half sisters who have taken DNA tests. We match but on some chromosomes in quite different ways. When Ancestry had their DNA Circles it showed my 2nd gg father William Furlsbury Carter and the hundreds of people who descended from him. I showed a genetic match with about half of them. The other half (through randomness of the DNA) no longer had enough common DNA to show a match.
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Robert, if the matches are second cousins or closer they will show as matches. The DNA matching is consistent to this point. After 2nd cousins, there may not be enough shared DNA to show a match.
Think of it this way, you have two kids sitting on the porch sharing a bag of M&M's. If they each grab a handful, they will not get the exact same colors, but they will have many of the same colors in their handfuls. If these same kids gave part of their candy to the next generation, there would be even less of some colors. (The colors represent different ancestors.) Each time the DNA is divided and mixed with the new parent, the DNA contains less of each generation of ancestors.
Each child gets about 50% of their DNA from each parent. Each previous generation went through the same process, so you only share about 25% with each grandparent. You share about 12 1/2 % with each great grandparent.
Beyond the second cousin range, the amount of shared DNA has decreased to a minimal amount. See this chart to see how much DNA you would share with other relatives: https://customercare.23andme.com/hc/en-us/articles/212170668-Average-percent-DNA-shared-between-relatives
RelationshipAverage % DNA SharedRange
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