Introduction
Linguistically, consanguinity is a term that is derived from two Latin words "
con" meaning common, or of the same and "
sanguineus" meaning blood, hence, referring to a relationship between two people who share a common ancestor or blood. In other words, consanguineous marriage refers to unions contracted between biologically-related individuals. In clinical genetics, a consanguineous marriage means union between couples who are related as second cousins or closer [
1,
2]. Among Arabs, this would include double first cousins, first cousins, first cousins once removed, and second cousins. Uncle-niece marriage is prohibited in Islam and so is absent among Arabs. In population genetics, consanguinity may also refer to unions of individuals with at least one common ancestor such as those occurring within population isolates, small towns, and tribes; intra-community or endogamous marriages. The custom of endogamy among individuals belonging to the same tribe (
hamula or
kabeela) is and has been strongly favored among Arabs, with the consequence of unequal distribution of founder mutations among the population. A large number of studies into the effects of consanguinity on health and disease have not taken such discrepancies into consideration.
Consanguinity in Arab Populations
Socio-cultural factors, such as maintenance of family structure and property, ease of marital arrangements, better relations with in-laws, and financial advantages relating to dowry seem to play a crucial role in the preference of consanguinity in Arab populations [
3]. Consanguineous marriages are generally thought to be more stable than marriages between non-relatives, though there are no studies to compare divorce rates of consanguineous and non-consanguineous marriages among Arabs. It is generally believed that the husband's family would side with the consanguineous wife in marital disputes since she is considered part of the extended family. When there are children with disabilities, more family members share in caring for these children. Unlike what is thought, consanguinity in the Arab World is not only confined to Muslim communities. Several other communities, including the Lebanese, Jordanian, and Palestinian Christian populations, have also practiced consanguinity, but to a lesser extent than Muslims [
4‐
7].
Consanguinity rates show wide variations among Arab countries, as well as within the same country (Table
1, Additional file
1). However, reports from Arab countries on consanguinity rates may sometimes include marriages between third cousins or far relatives within the consanguineous category. Although this discrepancy affects the total consanguinity rate, it does not markedly alter the average inbreeding coefficient. Therefore, for comparison of consanguinity rates among populations, two parameters are best used; the mean inbreeding coefficient (F) and marriages between first cousins. However, Arab societies have a long tradition of consanguinity, and the cumulative estimate of (F) may exceed the estimated value which is calculated for a single generation [
8].
Secular changes in the consanguinity rates have been noticed in some Arab populations. In Jordan [
9], Lebanon [
5], Bahrain [
10], and among Palestinians [
11‐
13], the frequency of consanguineous marriage is decreasing. Several factors may be playing a role in decreasing the consanguinity rates in Arab countries. Amongst these factors are the increasing higher female education levels, the declining fertility resulting in lower numbers of suitable relatives to marry, more mobility from rural to urban settings, and the improving economic status of families. Moreover, genetic diseases may be feared more now that infectious diseases are on the decline as causes of severe morbidity and mortality.
Generally, the highest rates of marriages to close relatives are consistently reported in the more traditional rural areas and among the poorest and least educated in society [
8]. Reports from some Arab countries have shown that consanguinity rates are lower in urban when compared to rural settings. Urban to rural first cousin rates in Algeria were 10% and 15% [
14], in Egypt, 8.3% and 17.2% [
15], and in Jordan, 29.8% and 37.9% [
6], respectively. Likewise the mean inbreeding coefficient was lower in urban as compared to rural settings in Syria (0.0203 versus 0.0265) [
16]. In Jordan, it was evident that the higher the level of education of the female partner, the lower the consanguinity rate. Only 12% of university educated females would marry their first cousins, whereas 25% of university educated males tend to marry first cousins [
6]. Similar trends of lower consanguinity rates among educated women, but not educated men, were noticed in Yemen [
17] and Tunisia [
18].
On the other hand, social, religious, cultural, political and economic factors still play roles in favoring consanguineous marriages among the new generations just as strongly as they did among the older generations, particularly in rural areas. Consanguinity rates seem to be increasing at a higher pace in Qatar [
19], Yemen [
17], the United Arab Emirates (UAE) [
20], and Tlemcen in Algeria [
14]. In Morocco, a study indicated an increasing consanguinity rate from the previous (21.5%) to the present (25.4%) generation [
21], while another study indicated a decreasing consanguinity rate [
22]. Consanguinity rates are not declining in some Arab countries because it is generally accepted that the social advantages of consanguinity outweigh the disadvantages [
23], and consanguinity is regarded as a deeply rooted cultural trend. It is believed that the practice of consanguinity has significant social and economic advantages. Consanguineous marriages among Arabs are respected because it is thought that they promote family stability, simplify financial premarital negotiations, offer a greater compatibility between the spouses and other family members, offer a lesser risk of hidden financial and health issues, and maintain the family land possessions [
3,
24,
25]. Among 390 women attending reproductive health clinics in Jordan, consanguinity was protective against violence during pregnancy [
26]. In all cases, reports on secular trends in consanguinity need to be treated with some caution because in countries where consanguinity is favored, major regional and ethnic differences in prevalence are commonly observed [
3].
Consanguinity and Reproductive Health
Research on the association of consanguinity with the different parameters of reproductive health in Arab countries is limited, both in quantity and in quality. Many studies fail to indicate clearly the different categories of consanguineous marriages in their methodology and thus the results are presented for consanguineous marriages as a single entity with the conclusions relying on a simple consanguineous versus non-consanguineous dichotomy. Given the wide range of F values in the 'consanguineous' group (F = 0.0156-0.125), with second cousin offspring (F = 0.0156) closer to non-consanguineous (F = 0) than to first cousins (F = 0.0625) or double first cousins (F = 0.125), such comparisons between consanguineous and non-consanguineous are thus not accurate. However, owing to the dearth of publications in the field among Arabs, this review will mention these studies with clear indication of the categories of consanguinity that are being compared.
Consanguineous marriages are widely practiced in several global populations, with some of the highest rates observed in the Arab World. Reports abound on both the negative and positive biological effects of consanguinity. In net terms, the reproductive criteria related to consanguineous versus non-consanguineous couples include earlier parental age at marriage, younger maternal age at first live birth, higher number of infants born to consanguineous parents, similar rates of abortions, and higher rates of postnatal mortality and birth defects in offspring of consanguineous parents. Furthermore, consanguineous unions lead to increased expression of autosomal recessive disorders. The CTGA Database on genetic disorders in Arab populations offers a clear evidence for a direct correlation between these two factors.
Studies on the association of consanguinity with chromosomal abnormalities such as Down syndrome and association with non-communicable disorders such as diabetes, hypertension, and psychiatric disorders among Arabs are presently non conclusive with the recommendation of performing standardized research in the future. Likewise, studies on the association of consanguinity with traits such as intelligence quotient and stature are scanty among Arabs and results of studies performed in Western countries cannot be applied directly to societies with high consanguinity rates such as the Arab society.
Scientifically, a considerable number of genes causing autosomal recessive conditions have been structurally and functionally determined at the molecular level through the joint collaboration of international and Arab scientists; these efforts should continue and expand given the high number of rare recessive disorders in the region.
Young Arabs contemplating marriage are nowadays seeking a scientifically sound answer to their questions: "Will our children be physically or mentally abnormal if I marry my cousin?" "How can we prevent having abnormal children?" Research on inbreeding is considered a priority in societies with high consanguinity rates to help understand and prevent the deleterious impact of consanguinity on health, and to provide standardized and evidence-based guidelines for health care providers to assist them in counseling for consanguinity.
Authors' contributions
GOT: Initiated the concept of the paper, collected partial data on consanguinity (Table
1 and additional file
1), made the illustration used in the paper, supervised all the primary text authoring written by co-authors in Dubai.
PN: Authored the review on the positive aspects of consanguinity and collected partial data on consanguinity (Table
1 and additional file
1).
TO: Authored the review on the negative aspects of consanguinity and collected partial data on consanguinity (Table
1 and additional file
1).
MTA: Facilitated the collection of published data on consanguinity by offering services available at government medical bibliographic facilities and reviewed the final version of the manuscript.
NA: Discussed and approved the primary text of the manuscript as prepared by the team of the Centre for Arab Genomic Studies in Dubai.
HAH: Enriched the primary content of the paper co-authored in Dubai with extensively detailed data (Table
1 and additional file
1), and modified, revised, and added text content in different sections of the manuscript and made it reach to the present level.