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If a buccal swab is taken from a cousin of a man. (ie his uncle's son) and it was
compared to that man's DNA in a sibling test, would it be possible for a false positive test result showing that they were half siblings?

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Yes it would.

The relationship estimator is based on the number of shared chromosomes. The normal range for a half-sibling is 1317cM to 2312cM. The normal range for a 1st cousin is from 553cM to 1225cM. It's very possible that a larger amount of DNA is shared between the two brothers (uncle and father), and that by chance a larger percentage is also shared between the cousins, bringing them to the half-sibling range.

It's possible, but not very likely though...

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Well, this would happen if the uncle and the father were identical twin brothers.

Otherwise, it's pretty rare -- not impossible, but rare -- for two "first cousins" to have so much DNA in common that they register as if they were half-siblings.

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