linguistic:translation
Genetic definitions

Genetically, consanguinity derives from the reduction in variation due to meiosis that occurs because of the smaller number of near ancestors. Since all humans share between 99.6% and 99.9% of their genome, consanguinity only affects a very small part of the sequence. If two siblings have a child, the child has only two rather than four grandparents. In these circumstances, the probability is increased that the child will inherit two copies of a harmful recessive gene (allele) (rather than only one, which is less likely to have harmful effects).

Genetic consanguinity is expressed as defined in 1922 by Wright with the coefficient of relationship r, where 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 (sharing 50% of DNA), siblings have a value of r =0.5, a parent's sibling has r =0.25 (25% of DNA), and first cousins have r =0.125 (12.5% of DNA). These are often expressed in terms of a percentage of shared DNA but can be also popularly referred to as % of genes although that terminology is technically incorrect.

As a working definition, unions contracted between persons biologically related as second cousins or closer (r ≥ 0.03125) 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 it is estimated that at least 8.5% of children have consanguineous parents.

In clinical genetics, consanguinity is defined as a union between two individuals who are related as second cousins or closer, with the inbreeding coefficient (F) equal or higher than 0.0156, where (F) represents the proportion of genetic loci at which the child of a consanguineous couple might inherit identical gene copies from both parents.

It is common to identify one's first- and second-degree cousins, and sometimes third-degree cousins. It is seldom possible to identify fourth-degree cousins, since few people can trace their full family tree back more than four generations. (Nor is it considered important, since fourth cousins tend to be genetically no more similar to each other than they are to any other individual from the same region.)