Global prevalence of consanguinity as cited by Bittles AH, Black ML (ref. 6: reproduced with permission from . net/index.php/Global_prevalence) 

... defined population often shares genes because of inbreeding and eventually consanguinity (derived from a Latin word ‘blood relation’), implying sharing of genetic heritage because of marriage between close relatives originating from a common ancestor (i.e., mating of relatives). With limited natural selection, recessive genes may become more frequent in an inbred compared with an outbred population. In European history, already the Roman civil law prohibited a marriage if the couple was within four degrees of consanguinity. Such prohibitions were also adopted by the church. However, the rules did not reach all of Europe, and consanguinity has been common particularly in cultural and geographic isolates. European nobility kept itself above the common law, and interrelated marriages were convenient in consolidating class, land and power. In many other cultures, rules were enacted against marriage between relatives, while in some others inbreeding has been and still is commonplace. In some large populations of Asia and Africa 20 to 50% of all marriages, and in certain areas of Pakistan most marriages have been consanguineous. 1 The reasons to practice inbreeding at a large scale include religion and culture, socio-economic class and royalty, and geographic isolation and small populations. Inbreeding is considered a problem because it increases the chances of receiving deleterious recessive genes (i.e., two mutated copies, alleles, required to cause disease) inherited from a common ancestor. Inbreeding coefficient is defined as the probability of receiving two copies (one from mother and the other from father) of the same ancestral gene, which are identical by decent. The coefficient is 1/16 for first cousins and 1/64 for second cousins. Persons with genetic diseases may be seriously handicapped and unable to breed. Dominant alleles (only one disease allele required to cause disease) often cause disease at early age, and the disease alleles tend to disappear from the population because of selection during con- secutive generations. Recessive diseases are insidious because carriers of single disease alleles are healthy and they may reproduce normally even though disease alleles are usually a selective disadvantage. Affected are only some children (by average 25%) of the parents, both of whom are carriers of the disease allele. In outbred populations, recessive disease alleles are usually rare because they are selected against over generations; thus, the likelihood of inheriting two recessive disease alleles is low. In inbred populations, the selection against deleterious alleles is less efficient because they are reintroduced into decedents a few generations later. The literature on human genetics is full of examples on rare genetic diseases in inbred populations, which were geographically isolated (e.g., Finns and French Canadians) or cultural distinct (Ashkenazi Jews). 1–4 Curiously, despite the ancient rules against consanguinity, the scientific evidence on the deleterious effects has been relatively recent. The first reports were published at the time of Charles Darwin in the mid-1800s. He had a personal reason to be upset and to demand scientific evidence because he married his first cousin. Darwin remained sceptical, and all his 10 children were healthy. In the present chapter, we discuss consanguinity in North Africa (NA) and other parts of the world, and some of its deleterious consequences in indigenous and immigrant populations. It should be noted that many immigrant communities, at least in Europe, are quite inbred partly because of their cultural isolation and partly because the immigrants constituted close relatives who then settled in their own communities. For example, in the UK, it is estimated that more than half of marriages of some Pakistani Muslim immigrants are between first cousins. NA is also an endemic area for deleterious recessive diseases, which are very common in the population, including sickle cell anaemia and thalassaemia. The mechanism for such recessive diseases was a puzzle to population geneticists, who were used to the paradigm that pathological conditions have a selective disadvantage. The measure relating to selection was ‘fitness’, and individuals with deleterious mutations would have less than optimal fitness. Infections, particularly before reproduction, were considered the main selective force. Sickle cell anaemia taught another paradigm of fitness and recessive diseases because the disease provided a survival advantage in regions where malaria was endemic. Consanguineous marriages have been practiced since the early existence of modern humans. At present, 20% of world populations live in communities with a preference for consanguineous marriage. 5 Consanguinity rates vary from one population to another depending on religion, culture and geography. Noticeably, many Arab countries display some of the highest rates of consanguineous marriages. 6 As can be seen from figure 1, an important cluster of countries with high levels of consanguinity is observed in most communities of NA, the Middle East and West Asia, a transverse belt that runs from Pakistan and Afghanistan in the east to Morocco in the west, and in South India, with intra-familial unions collectively accounting for 20–50+% of all marriages. 6 The highest consanguinity rates were reported among Pakistan army personnel and isolated Egyptian Nubians (76 and 80.4%, respectively). First cousin unions are especially popular, comprising 20– 30% of all marriages in some populations, in particular, the paternal parallel subtype in Arab societies. 6 Contrary to common opinion, consanguinity is not confined to Muslim communities. In NA and the Middle East, for example, marriages between relatives are also observed among Christians and Jews. It has been argued that consanguinity must be seen as a cultural rather than an Islamic or religious trait. Marriage choice and decision-making is a complex interaction of various social and cultural patterns of behaviour and norms. The main reasons for a preference for consanguineous unions are historical, cultural, socio-economic and geographical. Consanguineous unions are estimated to represent 40 to 49% and 29 to 33% of all marriages in Tunisia and Morocco, respectively. In Algeria, data from the 2007 survey showed that 39% of marriages in the sample population were between cousins. Similar estimates have been reported from the Tlemcen region of West Algeria. In Egypt, prevalence figures for consanguineous unions ranged from 20 to 33% across different studies and 39% as determined by the National Population Council. The prevalence of consanguineous unions also varies by place of residence in Egypt. It ranges from 25.4% in Lower Egypt to 55.2% in Upper Egypt. Estimates indicate that first-cousin marriages are approximately two-thirds of all consanguineous marriages in Morocco and 40% in Algeria. Close consanguinity accounts for 22% of the total marriages in Egypt and is higher in rural areas. A consanguinity rate of 32% with first cousin unions was observed in Tunisia. In Morocco, it (parallel and cross cousins) accounts for 42% of all consanguineous union. Similarly, in Egypt, the husband was more likely to be a relative from the father’s side than the mother’s side (14 and 8%, respectively). Surprisingly, it has been recently reported that unlike the previous generations, paternal first-cousin marriages in Algeria were lower than the paternal first cousin subtype in the new generation. Generally, the highest rates of marriages to close relatives are consistently reported in lower educational and socio-economic groups, the traditionally religious and the early married; the rates tend to decline with modernization; however, not in all populations. 6 Reports from North African countries have shown that consanguinity rates are lower in urban compared with those in rural settings. Urban to rural first cousin rates in Egypt were 8.3 and 17.2%, respectively. The highest level of consanguineous marriage was found in rural Upper Egypt. Likewise, the frequency of unions between relatives was lower in the urban than in the rural areas in Morocco (25.9 and 33.3%) and Algeria (30.6 and 40.5%). For Morocco immigrants, ethnicity (distinction Berber vs. Arab) is a better predictor of consanguineous marriage than the region of origin, as Berbers in Belgium are more often married to a relative than Arabs. The higher the level of education of the female partner, the lower is the consanguinity rate. In Egypt, for example, a woman’s chance of marrying a relative decreased from 35% among women who had never attended school to 23% among women with a higher education level. The comparable figure for Algeria was 58% in non-educated women and 7% in highly educated women. According to two different studies, only 6.3 to 7% of highly educated females would marry their first cousins, whereas 12 to 15% of highly educated males tend to marry first cousins. Similar trends of lower consanguinity rates among educated women, but not educated men, were noticed in Tunisia and Morocco. 7 In a Moroccan multinomial logistic analysis, considering variables education and residence, it was shown that women’s education was found to be the most significant variable in distinguishing kin groups from the outbred. The more urbanized the place was, the lower the consanguinity rate. Variable secular trends in the consanguinity rates have been noticed in most North African populations. In Algeria, for example, consanguinity rates are increasing in the current generation (from 32 in the old generation to 40% in the new generation), 8 while in others such as Egypt and Tunisia the frequency of consanguineous marriage may be decreasing. According to two Moroccan surveys from 1987 and 1992, the prevalence of consanguineous marriages declined by 4 percentage points. Amongst the contributing factors are the increasing higher ...