Genetic Definitions
The percentage of consanguinity between any two individuals decreases fourfold as the most recent common ancestor recedes one generation. Consanguinity, as commonly defined, does not depend on the amount of shared DNA within two people's genome. It rather counts the number of meioses separating two individuals. Because of the effects of pedigree collapse, this does not directly translate into the amount of shared genetic substance.
It is common to distinguish first-degree cousins, second-degree cousins, and often also third-degree cousins. Since comparatively few people can trace their full family tree for more than four generations, the identity of fourth-degree cousins often cannot be established. Also at a genetic level, half-fourth cousins typically do not exhibit greater genetic similarity with one another than with any other individual from the same population.
Double first cousins are descended from two pairs of siblings, and have the same genetic similarity as half-siblings.
Conventionally, genetic consanguinity is expressed with the coefficient of relationship r. r is defined as the fraction of homozygous due to the consanguinity under discussion. Thus, a parent and child pair has a value of r=0.5, siblings have a value of r=0.5, a parent's sibling has r=0.25, and first cousins have r=0.125.
As a working definition, unions contracted between persons biologically related as second cousins or closer (r ≥ 0.0156) are categorized as consanguineous. This arbitrary limit has been chosen because the genetic influence in marriages between couples related to a lesser degree would usually be expected to differ only slightly from that observed in the general population.
Globally, the most common form of consanguineous union contracted is between first cousins, in which the spouses share 1/8 of their genes inherited from a common ancestor, and so their progeny are homozygous (or more correctly autozygous) at 1/16 of all loci (r = 0.0625).
Read more about this topic: Consanguinity
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