Consanguinity

Consanguinity is the relationship between individuals descended from common ancestors, who are more likely to have specific mutant genes (alleles) in common than the general population.^[1] People descended from two common ancestors, such as a brother and sister, are said to be of the whole blood; if they share only one ancestor, they are of the half blood.^[2]

In clinical genetics, a consanguineous marriage is defined as a union between two individuals who are related as second cousins or closer, with an inbreeding coefficient equal to or higher than 0.0156, or about 1 chance in 60.^[3][a]The coefficient of inbreeding (COI) is a measure of the probability that two alleles in each individual are identical by descent from a common ancestor of the two parents.^[4]

It has been estimated that about one fifth of the global population live in communities with a preference for consanguineous marriages, mostly those in the Middle East, West Asia and North Africa, and their diaspora. Among those populations, one in three marriages are between cousins.^[3]

In the UK, the Marriage Act 1949Act of Parliament of the United Kingdom to regulate marriages in England and Wales. lays out the forbidden degrees of consanguinity for a marriage to be legally recognised, but does not concern itself with the legality of any sexual relationship between two related individuals.^[5] That is dealt with by the Sexual Offences Act 2003, which in section 64(2) prohibits anyone from sexual activity with their parent, grandparent, child, grandchild, brother, sister, half-brother, half-sister, uncle, aunt, nephew or niece.^[6]