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 percentage^{1}. 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(1/2^{n})

where n = the number of steps in a genealogy to
each common ancestor^{2}. 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 individuals^{2}.

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 parents^{1}.

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.