Marital union among close biological kin is not uncommon. Current data suggests that about 10% of global population is related as second cousins or closer. The highest rates of consanguineous marriages occur in south and west Asia, Middle East, and north Africa. Overall, it is commonly seen in immigrant populations and the rate varies from region to region depending upon many factors including education status. Surveys suggest that 20% of world population prefers consanguineous marriages because of several social, economic, cultural, regional, racial and historical reasons. People believe that these marriages lead to increased couple stability and compatibility, stronger family ties, less divorce and domestic violence rates, transmission of property within the family, preservation of culture and social status. It has been seen that in highly consanguineous regions there is a common and strong belief among people that marrying within the family reduces hidden uncertainties about health and financial issues.
From the genetic perspective these marriages may cause homozygosity of alleles in offspring preserving advantageous genetic traits. It restricts the diseased allele to remain within the family and prevents spread of diseased allele outside the family and may lead to its easy elimination from the gene pool. Fertility rates are also higher in consanguinity but the main reason for that may be lower age of marriage in highly consanguineous regions. Some evidence shows that these marriages may protect against certain diseases.
However, biological disadvantages supersede all the benefits as per present level of research and medical establishment. Consanguinity is an important risk factor for expression of autosomal recessive disorders in offspring. Blood relatives share same gene pool and act as carriers for genetic disorders, so inbreeding between close relatives highly favours transmission and expression of lethal genes in progeny leading to severe deleterious outcomes. The risk is closely related to degree of genetic relationship between parents, so risk is more where parents are close relatives. First cousins share about 12.5% of genome, second cousins about 3.2%, third cousins about 0.78%, and fourth cousins share about 0.2 %. So, first cousin marriages have maximum chances of transmitting the lethal genes to their progeny. Risk of transmitting genetic disease in consanguinity is much more than in normal population. The phenomenon of inbreeding increases the level of homozygosity for autosomal recessive disorders and leads to decreased biological fitness of population known as inbreeding depression. As consanguineous marriages may lead to transmission of lethal genes to progeny, they have many deleterious reproductive outcomes like congenital anomalies, cognitive impairment of vision and hearing problems, epilepsy, cystic fibrosis, hemoglobinopathies like sickle cell anemia and thalassemia, immuno-deficiency diseases, early onset of hypertension, cancer, still births, increased neonatal and infant mortality. Mortality in first-cousin pregnancy is about 3.5% higher than in non-consanguineous offspring, although other factors also play a role.
It has been found that more than 70% of infants with congenital anomalies die within first month of birth, whatever may be the cause of anomaly. Some studies show that unexplained infant mortality and undiagnosed severe conditions in high consanguineous populations may be closely associated with inbreeding in these populations. There is increasing evidence that highly consanguineous populations are more susceptible to certain infectious diseases like tuberculosis and hepatitis B.
As we have almost no treatment for genetic disorders, all the benefits are highly outclassed by disadvantages. So, its avoidance is highly advisable. People should be educated and made aware about deleterious consequences of consanguinity. Family history, pedigree analysis, pre-marital counselling and screening, preconception counseling, and genetic testing should be preferred in highly consanguineous populations to prevent the adverse health outcomes of consanguinity. Premarital and pre-conceptional screening should be made mandatory in highly consanguineous populations for early identification of carrier state. Media campaigns must be used as a tool to make people aware about it. Early detection and management of disorders related to it should be done and termination of affected pregnancy should be preferred. Advancement and rapid progress of newer modalities like next-generation sequencing may be an effective means of reducing deleterious outcomes of consanguineous marriages.
In conclusion, major scientific research does not favour consanguineous marriages and this is an important health concern which needs attention and further research.
The writer is a medical science student and a researcher from Srinagar. [email protected]