What is consanguineous marriage (CM)?
Cousin marriage, also known as consanguineous marriage (CM), is a sort of interfamilial union that is described as the union of two blood relatives who are second cousins or closer.
Social and economic considerations of consanguineous marriage
The preference for consanguineous marriage is influenced by social and financial factors, among other things. In terms of society, consanguineous marriage is thought to improve family bonds while also assisting in avoiding potential health or financial ambiguities that could arise from marriage with a spouse from a different family or culture.
Consanguineous marriages also make premarital planning much easier, and the bride’s relationship with her in-laws (and blood relatives) is inevitably closer and friendlier. A marriage arranged inside the family is an efficient way to reduce the financial costs in nations where either dowry (payment from the bride’s family to the groom) or bride-wealth (payment from the groom to the bride’s family) is the norm.
The current prevalence of consanguineous marriages
Consanguineous marriage is typically referred to in clinical genetics as a union between two people who are second cousins or closer. In other words, the parent-child pair inherits 1/32 of their genes from a common ancestor or ancestors, and on average, 1/64 of all loci in their offspring contain identical gene copies.
The majority of North and sub-Saharan Africa, West, Central, and South America, as well as Western Europe, North America, Australasia, and Russia are among the societies where the proportion of consanguineous marriages is less than 1%. Other societies where the proportion is between 1 and 10 percent include the Iberian peninsula, Japan, and South America.
Over 20% of the world’s population, including many of the most populated nations in Asia and Africa, lacks access to credible data on consanguineous marriage. Using Indonesia as an example, an examination of anthropological data revealed that 12 out of the 17 subpopulations investigated preferred or allowed first- or second-cousin marriages. As a result, it appears likely that a sizable but unidentified number of contemporary marriages in Indonesia have been consummated by biologically related partners. A similar situation may exist in many other nations where data are currently lacking.
Religion and consanguineous marriages
The majority of nations have long-held religious or cultural beliefs that inform their attitudes regarding consanguineous marriage, whether they are favorable or unfavorable. The most common type of consanguineous marriage is between first cousins, and it is universally accepted in Buddhism, Confucianism, Judaism, Islam, and Zoroastrianism/Parsi religion. Divergent viewpoints exist within Hinduism, with Dravidian South Indians favoring consanguineous unions while the Aryan tradition in North India forbids near-cousin unions. Christianity also has attitudes and beliefs that are in conflict.
First-cousin marriages must get diocesan dispensation in the Roman Catholic Church; in the Orthodox Church, the level of prescription is considerably stricter; nonetheless, first-cousin marriages are typically permitted in Protestant faiths without restriction.
First-cousin marriages between a man and his father’s brother’s daughter are preferred among Arab Muslim communities as opposed to the mother’s brother’s daughter pattern that South Indian Hindu people have established.
Clinical outcomes of consanguineous marriage
Following are some major clinical outcomes of CM.
Consanguineous marriage and prenatal losses
Contrary to the common and false idea that consanguineous unions diminish fertility, first-cousin kids have been found to have lower rates of pathological infertility. There has been conflicting information on fetal losses and stillbirths, with the majority of research finding no consanguinity-associated effect but also some instances of increased losses.
Because all of these studies rely on mothers’ recollections, which may not be reliable, and the levels of prenatal losses recorded are comparatively low to those obtained from studies using estimates of serum or urinary human chorionic gonadotrophin levels, it is important to proceed with some caution.
Several surveys have suggested that children born to consanguineous parents were smaller and lighter than other children, but other studies have not been able to detect a statistically significant difference. The data on the relationship between consanguinity and birth measurements are also contradictory.
Consanguineous marriage and early mortality
The early postnatal mortality rate is higher in the offspring of consanguineous unions, according to practically all research with acceptable sample sizes, but over time, estimates of the extra mortality associated with consanguinity have been significantly revised lower. This revision is likely the result of improved sampling methods and, the realization that earlier surveys yielded erroneously high values as a result of insufficient control for non-genetic variables.
According to the most recent representative mortality estimate, which is based on multi-national research of more than 600,000 live births and pregnancies, first-cousin children are 4.4% more likely than non-consanguineous children to die between the sixth and tenth months of pregnancy. However, even this number may be slightly inflated given that only a small number of the constituent polls in the analysis had expressly incorporated control for any confounding socio-demographic characteristics.
Consanguineous marriage and morbidity
Consang.net provides a comprehensive list of genetic disorders identified in the offspring of consanguineous couples. The diagnostic criteria used in studies on the prevalence of birth defects are particularly important because, in less developed nations, diagnoses may overlap with, and reflect, late fetal and neonatal survival rates. The large range in the excess incidence of birth abnormalities observed for first cousin progeny, is 0.7 to 7.5% greater than in non-consanguineous couples, with higher values recorded in more recent research.
There is a higher chance of recurrence and congenital abnormalities with complicated etiologies seem to be more prevalent in consanguineous families, probably reflecting unusual or even distinct recessive genes.
Consanguineous progeny have an elevated prevalence of many single-gene illnesses, such as autosomal recessive non-syndromal hearing loss, early onset retinal dystrophies-related blindness, childhood glaucoma, and anophthalmos and microphthalmos. Bilateral retinoblastoma has also been noted to be more prevalent in Saudi Arabia. In consanguineous kids, both mild and severe learning disabilities seem to occur more frequently and heightened rates of cerebral palsy have been found in the UK Pakistani population, with the causal recessive genes located on chromosomes 2q and 9p.
Besides elevated levels of α- and β-thalassaemia, rare complex hemoglobinopathies involving both α- and β-chain mutations in the same family, and other hematological disorders including coagulation deficiencies, have been reported at increased prevalence in consanguineous progeny. There also are elevated levels of acute lymphocytic leukemia and childhood cancers in some communities, and a wide range of inborn errors of metabolism have been diagnosed in different indigenous and migrant populations, including usually rare lysosomal storage disorders and cerebral lipidoses.
Factors that can eliminate this menace
Future generations’ consanguineous marriages are likely to face significant restrictions due to growing urbanization and a trend toward smaller families. Consanguineous marriage may still be regarded favorably by society, but there has been a significant shift in the trade-off between the positive social and economic effects of intrafamilial unions and their detrimental effects on health, particularly in families with multiple harmful recessive genes. This might also lessen people’s desire for close kin unions.
New diagnostic, counseling, and therapeutic techniques are required in conjunction with community education programs. Medical geneticists and genetic counselors differ in their understanding of the likely effects of consanguineous marriage, according to a US study. Comprehensive recommendations for the counseling of consanguineous couples and neonatal and childhood testing of their offspring have been established to address these inadequacies. In Western nations, hiring genetic counselors who are also ethnic community members may aid in fostering client confidence, especially when it comes to their preference for consanguineous unions.