Consanguineous Marriages

Prof. Rohan W. Jayasekara

Produced for the 125th Anniversary Celebrations of the Faculty of Medicine. Colombo. Sri Lanka. 1995

A marriage is said to be consanguineous when the union is between two people, genetically related, by descent, from a common ancestor. All human societies however primitive or geographically isolated, prohibit the mating of first degree relatives, namely the matings between parents and children and brothers and sisters(incest). Any marriage between relatives less close than sibs (brothers and sisters) or parents and offspring are not necessarily outlawed, but the dividing line between legal and illegal is hazy and varies somewhat between countries. In about one - half of the USA, uncle - niece and aunt - nephew (second degree) and first cousin (third degree) matings are forbidden by law. In most African societies consanguineous marriages are not allowed. In contrast in countries like Japan, India, Pakistan and the Middle East, upto 30% of marriages are consanguineous and are even encouraged as in Japan.

In Sri Lanka to quote the Legislative enactments of the Democratic Socialist Republic of Sri Lanka, Vol VI, Page VI / 210, Section 16 a, b and c, no marriage shall be valid - a) where either party shall be directly descended from the other ; or b) where the female shall be sister of the male either by the full or the half-blood, or the daughter of his brother or of his sister by the full or the half blood, or a descendant from either of them, or daughter of his wife by another father, or his son's or grandson's or father's or grandfather's widow ; or c) where the male shall be brother of the female either by the full or the half-blood, or the son of her brother or sister by the full or the half-blood, or a descendant from either of them, or the son of her husband by another mother, or her deceased daughter's or granddaughter's or mother's or grandmother's husband. It basically prohibits marriages which involve incest, those with step-brothers or step sisters, that between uncles and nieces and aunts and nephews.


We all carry blueprints for our characteristics in structures called genes which are segments of varying lengths of a molecule called DNA. Each and every structure and function of the body is under the control of a pair of genes in most instances. Each human body cell carries nearly 50 to 100,000 of these genes, as pairs, usually a pair for each trait. Of these about 4 to 8 are undesirable and faulty but as they are weak or recessive do not manifest the defect. The reason being that the other member of the pair is normal and powerful or dominant and thus suppresses the deleterious effects of the faulty one. For any of those 4 - 8 genes to have an impact, they need to be in identical pairs, which means that each parent has to contribute one member of the pair.


In consanguineous or close marriages, the ancestors are common, like for example in a first cousin mating, the grandparents are. In this instance the chances of a pair of these identical unfavourable genes meeting is far greater than in an unrelated marriage. Even in unrelated marriages both parents contribute the 4 - 8 unfavourable genes, which however have a lesser chance of being common, and there by seldom manifest the condition. Even in such unrelated matings the probability of producing a child with a serious birth defect or mental retardation is 3 - 4 %. This figure doubles in first cousin matings and trebles in uncle - niece and aunt - nephew matings.


The products of such close marriages tend to produce an increase of the various types of genetic disorders such as birth defects, mental retardation, deafness and blindness. Most pregnancies do terminate prematurely or as abortions or miscarriages, which in itself are signs of an unhealthy pregnancy, perhaps one carrying a defect. In view of the possibility of a vast array of disorders appearing following a consanguineous mating, it is impossible even to hazard a guess as to what conditions are more prone to appear. This reason limits the use of prenatal diagnosis or diagnosis of a possible defect or disorder in early pregnancy. However ultrasound scans and magnetic resonance imaging which are non - invasive diagnostic techniques may help detect a few gross physical malformations.

The main thrust towards a reduced frequency of consanguineous matings remains in the field of preventive genetics in the form of general education of society and specific programmes such as preconception counselling and genetic counselling. In Sri Lanka a very strongly held belief distinguishes the three types of first cousin matings and categorises the one between the children of two brothers as the worst while the others are harmless. This is not so with all three matings having the same risk of recurrence. It is this type of myth that needs to be demolished and the record set straight.


The most appropriate and convenient time to instill the importance of preventing close marriages is the adolescent period and therefore it will be useful for every school to be alive to this problem and take the necessary steps to advise them. Parents must be advised of the dangers of close associations of their children with the children of their brothers and sisters during their adolescence. The need to forge a close and friendly relationship and rapport with one's children at this stage, and giving them the right advise will play a crucial role in the reduction of the frequency of such close marriages.