Consanguinity means blood relationship by descent from the same ancestor, and not by marriage or affinity, and a consanguineous marriage is one contracted between biologically related individuals. In the main, the detrimental health effects associated with these marriages are caused by the expression of rare, recessive genes inherited from a common ancestor or ancestors. Many genetic diseases are recessive, meaning only people who inherit two abnormal genes for the same disease, one from each parent, will develop the disease. Since close relatives have more genes in common than unrelated individuals, there is an increased chance that parents who are closely related will carry the same disease genes and thus have an affected child. The rate of congenital malformations among the offspring of consanguineous marriages is approximately 2.5 times higher than that among the offspring of unrelated parents, and first cousin consanguinity has been shown to be significantly associated with an increased risk of various disorders.
People who share a recent common ancestor share more than 99.5% of their DNA; the closer the relatives are the more DNA they share. The amount of genetic material shared by first cousins is four times higher than that shared by second cousins. Once the relationship is between fourth cousins, the original amount of shared DNA reverts to the basic amount of 99.5%. Unions between individuals biologically related as second cousins or closer are categorized as consanguineous. Chapter 1 gives a general overview of consanguinity, including the background and history. There are also sections that discuss the legal, religious and biological (genetic) aspects.
The frequency of consanguineous marriages varies from one population to another. Consanguineous marriage is not restricted to specific religions or to population isolates, but is a long-standing practice in many regions of the world and it continues to be preferred by many populations, with more than 1,000 million people living in countries where between 20% and 50+% of marriages are consanguineous. The highest levels occur in the northern part of Africa, the Middle East and parts of Asia, whereas in what is described as the "Western world" – i.e. the entire North American continent, the whole of Europe (with the exception of Spain), Australia and New Zealand – the frequency is less than 1%. Chapter 2 gives a detailed account of the prevalence in the different parts of the world.
Chapter 3 discusses general health topics associated with consanguinity, genetic disorders and congenital malformations, and also describes the benefits and advantages that can accrue as a result of consanguineous marriages. These include socioeconomic advantages, the main ones of which are preservation of property, especially land, and the desire to keep this within the family, and the popular belief that in intrafamilial marriage, it is more likely that the bride will be compatible with her husband’s family, and the bride herself finds it reassuring to marry into a known family background.
Although it is well known that offspring of consanguineous parents have an increased risk for monogenic autosomal recessive diseases, the contribution of parental consanguinity to the development of common multifactorial diseases is controversial. Most of the common diseases are multifactorial in etiology, i.e. the disease will manifest only after the risk factor level, both genetic and environmental, has exceeded a certain cut-off point. Chapter 4 explores the association between consanguinity and susceptibility to common diseases.
Chapter 5 investigates the contribution of consanguinity to reproductive issues and fertility. While it used to be generally believed that inbreeding resulted in detrimental effects on reproductive outcome, some studies have shown only a moderate to slight impact. The fertility of consanguineous couples has also been extensively studied and numerous reports have concluded that consanguinity is not associated with either a significant positive or negative effect. However, in general, higher total fertility rates have been reported in consanguineous marriages. Similarly, reports regarding the association of consanguinity and fetal wastage have been conflicting, with some reporting that the total prenatal losses were essentially the same for consanguineous and non-consanguineous couples.
Awareness of the issues and problems associated with consanguineous marriages is the subject of Chapter 6. In many traditional Arab societies, in which the frequency of consanguineous marriages is very high, there is generally a low level of genetic literacy both among the public and most health care providers, and therefore the need for education is of vital importance in such communities in order to be able to establish programs aimed at reducing the rates of these marriages. The main factor in establishing such educational and counseling programs is to identify the target group(s) who would benefit most from such programs. Several studies carried out in different countries with high rates of consanguineous marriages are described; the purpose of these was to assess the levels of awareness among various groups, both general public and health care professionals, regarding the health problems associated with such marriages. The information gained from these surveys was used in each case to establish educational and counseling programs geared to that specific society. The results of various surveys that explored acceptance of prenatal diagnosis and termination of pregnancy among different populations and also attitudes to consanguinity are discussed.
Chapters 7 and 8 discuss future strategies for reducing both the frequency of consanguineous marriages and the number of affected children born to consanguineous parents. Chapter 7 continues the theme of Chapter 6 and details the requirements for the establishment of educational and counseling programs. Attempts in various countries to offer training, educational and counseling programs aimed at reducing the incidence of consanguineous marriages are described. The question as to whether religious intervention to discourage the practice of consanguineous marriage would be effective is also raised and an overview is given of the general trend of a decline in the worldwide rates of consanguineous marriages.
Chapter 8 concentrates on the genetic aspects, including genetic counseling and screening and a discussion of genetic testing and molecular analysis. The methods and techniques used for the identification of disease-related genes in consanguineous populations are described; once the causative gene for a specific disease has been identified, carrier screening in the specific community can be offered and prenatal diagnosis carried out in the case of carrier couples. Termination of pregnancy can then be offered in the case of an affected fetus. Preimplantation genetic diagnosis, which is a technique used to analyze embryos genetically before their transfer into the uterus in order to enable only unaffected embryos to be transferred, is also discussed.
Lutfi A. Jaber
The Bridge to Peace Community Pediatric Center
Institute of Neurology
Schneider Children's Medical Center of Israel
Sackler Faculty of Medicine
Tel Aviv University
Gabrielle J. Halpern
The Raphael Recanati Genetic Institute
Rabin Medical Center