How related should you expect relatives to be?

Like many Americans in the year 2018 I’ve got a whole pedigree plugged into personal genomic services. I’m talking from grandchild to grandparent to great-aunt/uncles. A non-trivial pedigree. So we as a family look closely at these patterns, and we’re not surprised at this point to see really high correlations in some cases compared to what you’d expect (or low).

This means that you can see empirically the variation between relatives of the same nominal degree of separation from a person of interest. For example, each of my children’s’ grandparents contributes 25% of their autosomal genome without any prior information. But I actually know the variation of contribution empirically. For example, my father is enriched in my daughter. My mother is my sons.

The sample principle applies to siblings. Though they should be 50% related on their autosomal genome, it turns out there is variation. I’ve seen some papers large data sets (e.g., 20,000 sibling pairs) which gives a standard deviation of 3.7% in relatedness. But what about other degrees of relation?

I didn’t find empirical data on that (imagine assembling a dataset with large numbers of known third cousins…perhaps in Iceland), but I did find this paper, Variation in actual relationship as a consequence of Mendelian sampling and linkage that was useful. The authors modeled the expectation and variance (and so standard deviation) of identity by descent, genomic relatedness. One of their models gives 3.84% standard deviation for siblings, so that seems pretty close to the empirical mark. Here is a table I put together from a subset of their results:

RelationshipRelatednessStandard Dev
Parent-child0.50
Full sibling0.50.0384
Grandparent-Grandchildren0.250.0251
Uncle-Aunt/Nephew-Niece0.250.0251
Cousin0.1250.0241
2nd Cousin0.03120.0117
3rd Cousin0·00780·0054

The distribution of relatedness among siblings seems about normal. So there are individuals who are less than 40% related to their “full-sibling” while others are more than 60% related. Notice that when it comes to third cousins the variation in expected relatedness is in the same range as expected relatedness. Some “3rd cousins” won’t share any genomic relatedness as defined by identity by descent from recent ancestors.

Related: How much of your genome do you inherit from a particular grandparent?

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5 thoughts on “How related should you expect relatives to be?

  1. My wife just got a hit on 23andMe who shared 6.9 of her dna with my wife. That strikes me as a first cousin once removed, but tracing back the connections, the relationship seems more distant – second cousins at best. Are there other possibilities?

  2. I was wondering if these averages changed depending on the population and its historical marriage practices? For example, Sicilians, Pakistani Muslims or Ashkenazim compared to Mexican Mestizo or African-Americans?

  3. Why the zero standard deviation for parents/children? The x chromosome is larger than the y chromosome, so shouldn’t male children be marginally more related to their mothers than to their fathers?

  4. Why the zero standard deviation for parents/children? The x chromosome is larger than the y chromosome, so shouldn’t male children be marginally more related to their mothers than to their fathers?

    i stipulated *autosomal genome* in the post. that excludes X or Y

  5. I was wondering if these averages changed depending on the population and its historical marriage practices? For example, Sicilians, Pakistani Muslims or Ashkenazim compared to Mexican Mestizo or African-Americans?

    the inbreeding has to be recent. so yes in the case of pakistan muslims. less so other groups.

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