Is testing one sibling enough? Practical Autosomal Genealogy

I was recently asked if it was enough to test on of a set of siblings for autosomal DNA. The quick answer is that you should always test at least three siblings and any living parents. Let’s look at the reason why.

As a review, autosomal DNA is the DNA that we all get equally from both parents. We get 50% from our mother and 50% from our father.

A family with five children
A family with five children

In the above example, Thomas and Ruth are the parents of five children. They are Mark, Peter, Matthew, Jane, and Beth. Thomas is dead. This is indicated by the line through his icon.

We should test Ruth because she is the oldest member of the family group. She has 100% of her own DNA. We cannot test Thomas, as he is dead. Thus, to learn about Thomas’ DNA relatives, we need to test his children.

Testing the first child will tell us about 50% of Thomas DNA and the child will match about 50% of the relatives that Thomas would have matched.

Mark has half of Thomas’ DNA and will have about half of Thomas’ potential matches.

Although each sibling shares about half their DNA with each other and their parents, each one gets a different mix of DNA. If Mark and Jane test, about 1/2 of the matches to their father’s side will be shared between them. On average, testing two siblings results in matching to about 75% of the matches from the untested parent. However, there is a range. Thus, some may have more and others less.

There is DNA that both Mark and Jane got from Thomas, DNA that only one of them got from Thomas, and DNA that nobody tested got from Thomas.

If three siblings test: Mark, Jane, and Beth, more of their father’s DNA will be represented. About 87% of Thomas’ DNA and his DNA matches will be represented by at least one of the tested children.

As more of Thomas’ children are tested, the amount of his DNA that is unrepresented goes down.

Thus, testing at least three children ensures that most of the potential relatives are identified. Because autosomal DNA is inherited randomly, this may be more or less than 87%.

For those who like formulas, the amount of tested DNA follows this pattern 1/2 + 1/4 + 1/8 + 1/16…

3 thoughts on “Is testing one sibling enough? Practical Autosomal Genealogy”

  1. Thanks for the explanation. I’m new at this – how do you take the created(?) DNA for Thomas and use it to find Thomas matches?

    1. For most websites (FTDNA, Ancestry, 23andMe, or MyHeritage), you compare the matches of the children to the matches of their mother. When someone matches a child but not their mother, you know the match came from the father. There are more advanced tools at GEDMatch Genesis, but I need to write tutorials on those.

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