How often do two half relatives share DNA?

Following up on the last post about full relatives, the plot below shows the rates that half relatives share ≥ 7 cM IBD segments. This uses the same abbreviations as in the previous post—1C means first cousins, 3C1R stands for third cousins once removed—but all relationships are prefixed with an `h’ for half. The first relationship, hAV, is half-aunt/uncle-niece/nephew. Papers often refer to aunt/uncle and niece/nephew relatives as “avuncular,” so the plot uses AV as an abbreviation.

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The rates are very similar to the “roughly” equivalent full relatives from before (see the table of equivalent relationships), but are a bit lower here. For example, half-third cousins (h3C) share at least one ≥ 7 cM segment in 71.1% of pairs versus the rate in third cousins once removed (3C1R—a roughly equivalent full relationship) of 72.7%.

Details of the simulation are the same as in the last post. This includes using the same number of pairs (100,000) for each relationship type.

Some have asked about rates for different minimum segment lengths. This is perhaps best to represent in a tool, and we’ll work on getting one up in the coming weeks.

Amy WilliamsAmy Williams is a Senior Scientist at 23andMe working on the population genetics research & development team. Prior to joining 23andMe in 2022, she was an associate professor of Computational Biology at Cornell University. The opinions and tools on this website are strictly those of the author.
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