# Dictionary Definition

relatedness n : a particular manner of
connectedness; "the relatedness of all living things" [ant:
unrelatedness]

# User Contributed Dictionary

# Extensive Definition

In population
genetics, Sewall
Wright's coefficient of relationship or coefficient of
relatedness or relatedness or r is the probability that at a random
locus,
the alleles there will be
identical
by descent or autozygous.

For example, in a diploid species (e.g. all
mammals), the coefficient of relationship between parent and
offspring is 0.5, meaning half of the offspring's genome comes from
that parent. Or, more precisely said, half the genomes are
identical by descent.

Some other relatedness examples: (assuming no
consanguinity, for
diploid species unless noted otherwise)

Some of these definitions of r are tricky to
calculate. For example, consider that you are one of a family of
half brothers with the same mother but different fathers. Call rm
the coefficient of matrilineal relatedness and call rp the
coefficient of patrilineal relatedness. What is the coefficient of
relatedness to each of your half brothers?

rm=0.5 as it does for normal siblings, since
there is a 1 in 2 chance that you and your half brother developed
from eggs from harboring the same haploid genome selected during
maternal meiosis. But rp=0 since there is no chance that any two
brothers inherited the same allele from different fathers by
descent. The average of these rs equals 0.25, as we see above. You
and your half brother are one quarter genetically identical by
descent.

In the example above, the differentiation between
maternal and paternal coefficients of relatedness was made in order
to introduce the phenomenon of imprinting, or parent-specific gene
expression into the calculation of r.

To illustrate how an imprinting turns normal
siblings into half siblings, consider that you are one of a cadre
of full siblings. What is your coefficient of relatedness to one of
your siblings at an imprinted locus in a maternally inherited
allele? (Maternally inherited alleles are called "madumnal," and
paternally inherited alleles are called "padumnal".)

The value of r at this imprinted locus is equal
to the value of r for the entire genome of the half brothers. For
the brothers above, rm=0.5 and rp=0. For the madumnal locus in the
family of full siblings rm=0.5 too, since all expression is from
the mother's allele. Normally, rp=0.5 for full siblings with the
same father, but the effect of imprinting is to negate paternal
expression, hence rp=0. Therefore you and your full siblings
resemble half-siblings at imprinted loci.

## See also

## References

- Sewall Wright, (1922). Coefficients of inbreeding and relationship. American Naturalist 56:330-338

relatedness in Portuguese: Coeficiente de
parentesco