consanguinity

n. kinship, blood relation, connection through common ancestry

Consanguinity ("con- (with) (blood) -ity") refers to the property of being from the same genetic lineage as another person. In that respect, consanguinity is the quality of being descended from the same ancestor as another person. Consanguinity is an important legal concept in that the laws of many jurisdictions consider consanguinity as a factor in deciding whether two individuals may be married or whether a given person receives property when a deceased person does not leave a will.
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(n.)
The relation of persons by blood, in distinction from affinity or relation by marriage; blood relationship; as, lineal consanguinity; collateral consanguinity.
  

Everyone carries rare recessive alleles, rare genes that are generally innocuous in the heterozygous state but that in the company of another gene of the same type are capable of causing an autosomal recessive disease. We are all reservoirs for genetic disease. First cousins, as noted, share a set of grandparents. So for any particular allele (gene) in the father, the chance that the mother inherited the same allele from the same source is 1/8. Further, for any gene the father passes to his child, the chance is 1/8 that the mother has the same gene and ½ that she transmits that gene to the child so 1/8 X ½ = 1/16. Thus, a first-cousin marriage has a coefficient of inbreeding of 1/16. The added risks for first cousins depend not only upon this coefficient of inbreeding but also upon their genetic family histories and, in some cases, upon test results (for example, for beta thalassemia for first cousins of Italian descent). However, there are always added risks from the mating of closely related persos and those risks are not negligible.

The relation subsisting among all the different persons descending from the same stock or common ancestor. Some portion of the blood of the common ancestor flows through the veins of all his descendants, and though mixed with the blood flowing from many other families, yet it constitutes the kindred or alliance by blood between any two of the individuals. This relation by blood is of two kinds; lineal and collateral.

Lineal consanguinity is that relation which exists among persons, where one is descended from the other, as between the son and the father or the grandfather, and so upwards in a direct ascending line; and between the father and the son or the grandson, and so downwards in a direct descending line. Every generation in this direct course makes a degree, computing either in the ascending or descending line. This being the natural mode of computing the degrees of lineal consanguinity, it has been adopted by the civil, the canon, and the common law.

Collateral consanguinity is the relation subsisting among persons who descend from the same commnon ancestor, but not from each other. It is essential to constitute this relation, that they spring from the same common root or stock, but in different branches. The mode of computing the degrees is to discover the common ancestor and, beginning with him to reckon downwards, so the degree the two persons, or the more remote of them, is distant from the ancestor is the degree of kindred subsisting between them. For instance, two brothers are related to each other in the first degree because from the father to each of them is one degree. An uncle and a nephew are related to each other in the second degree, because the nephew is two degrees distant from the common ancestor, and the rule of computation is extended to the remotest degrees of collateral relationship. This is the mode of computation by the common and canon law. The method of computing by the civil law is to begin at either of the persons in question and count up to the common ancestor, and then downwards to the other person, calling it a degree for each person, both ascending and descending, and the degrees they stand from each other is the degree in which they stand related. Thus, from a nephew to his father is one degree; to the grandfather, two degrees and then to the uncle, three; which points out the relationship.

The mode of the civil law is preferable, for it points out the actual degree of kindred in all cases; by the mode adopted by the common law different relations may stand in the same degree. The uncle and nephew stand related in the second degree by the common law, as are two first cousins or two sons of two brothers. But by the civil law the uncle and nephew are in the third degree and the cousins are in the fourth. However, the mode of computation is immaterial as both will establish the same person to be the heir.

This entry contains material from Bouvier's Legal Dictionary, a work published in the 1850's.