Two Neill Cousins, Shared DNA, and Coin Flipping

Shared amounts of autosomal DNA can vary from one relationship to another. One should not assume that just because the amount of shared DNA is less that the individual is more distantly related when the paper genealogical tree is viewed. The amount of DNA shared can vary somewhat even if two different sets of individuals have the same paper tree genealogical relationship.

  • M-my third cousin through our descent from Samuel Neill (died 1912 Hancock County, Illinois). M and I share 14 cM across two segments.
  • C-my third cousin once removed through our descent from John Neill (born 1810s Ireland). C and I share 105 cM across five segments. John is the father of Samuel Neill (died 1912).

The amount of shared DNA for individuals with a known relationship should fall within ranges of values that have been established. Average amounts of shared DNA are just that: averages.

It’s similar to tossing twenty coins and counting the number of tails that are obtained. Sometimes you’ll get ten tails, sometimes you’ll get five tails, and sometimes you’ll get fifteen. It doesn’t mean that you don’t have twenty coins or that the coins are not fair. It’s just how probability works. The same applies to the amount of shared autosomal DNA between two individuals.


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