consanguinity: one construct, many systems

In anthropology, consanguinity is relatedness through common descent (the 'blood' tie), as opposed to affinity, the tie created by marriage, and it is the backbone of how societies name and organise kin. Morgan's Systems of Consanguinity and Affinity of the Human Family compiled kinship terminologies worldwide and showed that peoples sort consanguineal relatives into systematically different categories. The referent is the structure of descent-based kin relations, mapped through terminology and genealogy, and it is the same underlying idea of shared ancestry that population genetics and clinical genetics later made quantitative.[1]

Unit

The categorised degree of descent-based relatedness between kin

Recovery means

Not applicable

Measured by

Mapped through kinship terminologies, genealogies, and the degrees of relation a society recognises

In population genetics, consanguinity is the same shared-ancestry idea turned into a number: Wright's 1922 Coefficients of Inbreeding and Relationship defined the coefficient of relationship as the expected fraction of genes two individuals share through common ancestors, and the related inbreeding coefficient as the probability that an individual's two alleles at a locus are identical by descent. The referent is a probability of shared genes computed from a pedigree, and it is the quantitative form of the anthropologist's degree of kinship, the same construct expressed as a measurable fraction of the genome.[2]

Unit

A probability (0–1) that alleles are shared by common descent

Recovery means

Not applicable

Measured by

Computed from pedigrees as the coefficient of relationship or inbreeding coefficient, or estimated genomically from shared segments

In clinical genetics, consanguinity names the marriage of close biological relatives and matters because shared ancestry raises the chance that a child inherits two copies of the same recessive allele, increasing the risk of autosomal-recessive disease. Bittles's 2001 Consanguinity and Its Relevance to Clinical Genetics reviews how this elevated homozygosity translates into measurable excess risk and how it is counselled in practice. The referent is the clinical consequence of descent-based relatedness, read directly off the inbreeding coefficient, and it is the same shared-ancestry construct that anthropology categorised and population genetics quantified.[3]

Unit

The excess risk of recessive disorder tied to the degree of relatedness

Recovery means

Not applicable

Measured by

Assessed from the inbreeding coefficient of a union and the observed excess incidence of recessive disorders in offspring