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The relatedness coefficient (r) is a measure of the biological relationship between two individuals.  Values for r range from 0 to 1 and are expressed as a percentage1.  Values that approach 1 indicates a higher level of inbreeding while values that approach 0 indicate two individuals with only very distant common ancestors.  The coefficient is calculated as

r = sum(1/2n)

where n = the number of steps in a genealogy to each common ancestor2.  In the case of siblings, each have two common ancestors (each parent).  To connect between each parent requires two steps (sibling to parent to sibling).  Thus, the coefficient between the two siblings is:

r = (1/2)2 + (1/2)2

and r = 0.5.  To calculate more complicated relationships, accurate family trees must be calculated to determine the number of paths and steps between individuals2.

The term is often used in a genetic framework and can be used to determine the likelihood that two individuals share common alleles.  In humans, this is important in terms of determining the likelihood of two individuals passing a shared deleterious allele inherited from a recent common ancestor.  If two individuals are closely related, such as cousins, the likelihood that they share the same deleterious allele increases and increases the risk that offspring will inherit copies from both parents1.

References

1. Wyttenback, R. 2012. Relatedness.  Retrieved March 24, 2015 from http://ess.nbb.cornell.edu/relatedness.html.

2. Lancaster, F.M. 2015. Calculation of the coefficient of relationship.  Genetic and Quantitative Aspects of Genealogy.  Retrieved March 24, 2015 from http://www.genetic-genealogy.co.uk/Toc115570135.html.

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