consangunity

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970 www.thelancet.com Vol 367 March 25, 2006
Consanguinity: implications for practice, research, and policy
Consanguinity,orclose-kinmarriage,hasbecomeasource
of major scientific and public interest worldwide with the
remarkable advances made in genetics over the past
50 years. WHO defines consanguineous marriage as one
between individuals who are second cousins or more
closely related. However, lesser degrees of consanguinity,
which are relevant to pregnancy outcomes, have been
documented, especially in highly inbred families.
In the Middle East, where consanguinity is common-
place, the recent wealth resulting from the discovery of oil
has led to great improvements in childhood nutritional
status and efforts to combat infectious disease, which in
turn have resulted in genetic disorders garnering ever-
increasing attention. This region of the world is largely
inhabited by Arabs and Muslims who traditionally favour
close-kin marriage. Excluding Israel and Cyprus, the
consanguinity rates range from 20% to 70% in most
Middle-Eastern countries.1,2
Although not a religious
prescription, many might perceive the practice of
consanguinity as affected by religion.3
In fact, it is deeply
rooted in the Arab culture and has been practised over
many generations not only by Muslims, who generally
prefer marriages between first cousins, but also by
Christian Arabs, albeit with lower rates and with a
tendency towards second-cousin marriage.1
Factors that
promote consanguinity in these societies include
economic reasons, revolving around the preservation of
wealth (land in particular), psychosocial advantages, and
geographical and tribal traditions.
Despite their perceived social advantages, consan-
guineousmarriagesareassociatedwithahigherfrequency
of autosomal recessive disorders (because of homo-
zygosity by descent) and multifactorial birth defects than
are marriages between unrelated individuals. These
disadvantages translate into excess prenatal, perinatal,
and postnatal mortality, as seen in the Arabian Peninsula
in many endogamous communities with a tendency
towards large family size.4
Generally, we counsel that
a first-cousin couple has a two-fold higher risk of
having a child with a major birth defect than a non-
consanguineous couple. There is no clear association
between parental consanguinity and the frequency of
common diseases such as diabetes mellitus, asthma, or
adult-onset behavioural and psychiatric disorders,
although some studies have shown an adverse effect on
cognitive performance.5,6
The effect of consanguinity on
reproductive potential is controversial, with some studies
reporting a decrease in fertility7
and some an increase.8
What is clear is that there is an increase in fetal wastage
(ie, abortions, stillbirths, and other forms of pregnancy
wasting) in consanguineous marriages.8
This knowledge
has led to increased awareness of the negative effect that
consanguinity has on health, resulting in a trend of decline
in some areas.9,10
The tradition of marrying a relative,
however, remains common in the Middle East.
One of the scientifically useful features of consanguinity
is that it facilitates homozygosity mapping of disease-
related genes. Homozygosity mapping in inbred families
or isolated populations is a practical strategy for mapping
the genes of rare autosomal recessive disorders.11,12
The
high prevalence of consanguinity in a population makes it
possible to identify inbred families with rare recessive
disordersthatarelargeenoughtoindependentlyestablish
linkage. In these families, individuals with rare recessive
traits are likely to have inherited both copies of the
mutated gene from a common ancestor, and are thus
identical by descent not only at the trait locus but also at
neighbouring marker loci. Homozygosity mapping
overcomes two major obstacles to gene discovery: locus
heterogeneity and variable expressivity. The successful
mapping of genes can be used to develop diagnostic tests
for carrier identification and for prenatal diagnosis,
especially for prevalent disorders in which consanguinity is
a key factor. As such, funds allocated for research activities
in the specialty of human and medical genetics in the
Middle East should be increased.
To overcome the health burden imposed by con-
sanguinity, preventive strategies should focus on several
factors. One such factor relates to public education on
genetic diseases and the variables that contribute to their
increased frequency. Basic information on this topic
shouldbeincludedaspartofsecondaryschoolcurricula,to
emphasise the effect of consanguinity and also to deal
with applicable preventive measures, making available
carrier-testing for common disorders in the community
along with appropriate genetic counselling. Premarital
and preconception testing and counselling for common
disorders, preimplantation genetic diagnosis, and prenatal
diagnosis and termination of pregnancy (within the
allowed limits in religion) should also be integral parts of

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the strategy to reduce the prevalence of autosomal
recessive disorders and birth defects. The application of
this strategy, however, should be accompanied by careful
and well-informed counselling to avoid stigmatisation.
The promotion of scientific research efforts for the
development of methods for genetic testing for
population-specific consanguinity-related disorders is
another important strategy.
*AhmadSTeebi,HatemIEl-Shanti
Division of Clinical and Metabolic Genetics, Hospital for Sick Children
and University of Toronto, Toronto, Ontario, ON M5G 1X8, Canada
(AST); and Division of Medical Genetics, Children’s Hospital of Iowa
and University of Iowa, Iowa City, Iowa, USA (HIE-S)
ahmad.teebi@sickkids.ca
We declare that we have no conflict of interest.
1 Teebi AS, Farag TI, eds. Genetic disorders among Arab populations, 1st edn.
New York: Oxford University Press, 1997.
Why the modern Middle East?
The Middle East as a geopolitical and cultural concept is a
construct that goes beyond the confines of geography.1
The WHO office responsible for this part of the world
describes its remit as the eastern Mediterranean, which,
somewhat surprisingly, includes Morocco, the most west-
ern country of the Mediterranean, and Pakistan, which is
some 2000 miles away from its eastern shores.2,3
Many
definitions of the Middle East (sometimes called near
east) exist; for the presentpurposes, the term denotes the
Arab countries of north Africa, the Levant, and the
Arabian Peninsula, as well as Turkey and Iran.4
This region stretches from the shores of the Atlantic
roughly to the banks of the Oxus. A great variety of peo-
ples speaking many languages and dialects inhabit the
lands of the Middle East, ranging from Berbers and Tuareg
to Arabs, Persians, and Turks. To understand the diversity
and the unifying characteristics of the Middle East, we
have to look at the past behind the present.
Mesopotamia, “the land between the rivers” Tigris and
Euphrates in Iraq, and Egypt both vie for acknowledg-
ment as the cradle of civilisation; they produced the
ancient civilisations of the Pharaohs, Sumerians,
Babylonians, and Assyrians, whilst the Fertile Crescent
witnessed the rise of the three Abrahamic faiths of
Judaism, Christianity, and Islam, all professing a strict
monotheism. Yet, despite the successive empires and dif-
ferent religions, the territories of the modern Middle East
were once unified politically and religiously, during the
classic age of Islam.5
When, after the death of the Prophet Muhammad in
632, the Muslim armies conquered large parts of north
Africa, Spain, southern Italy, the Levant, Asia minor, and
Persia, they brought with them their religion, Islam, and
their language, Arabic.6
Because of the relative tolerance
of the Muslim creed, which afforded the so-called “People
of the Book”, such as Jews and Christians, protected sta-
tus, it took many generations until the majority of the
subjugated population became Muslim and spoke Arabic
as their native tongue. Yet, by the 9th century, the Arabo-
Islamic civilisation held such an attraction that both a
Christian bishop in Spain and a Karaite Jew in Palestine
could lament the fact that their coreligionists studied
Arabic rather than Latin or Hebrew, respectively.7,8
Over the later medieval period, the Muslim world splin-
tered into several empires, such as those of the Fatimids
and Mamluks, the Seljuqs and Mongols, with the
Ottomans emerging as a major player whose military
might was illustrated by the sack of Constantinople in
1453 and the sieges of Vienna in 1529 and 1683. Except
forPersia,whichwasruledbytheSafavidandQajardynas-
ties, the Ottomans dominated the Middle East until the
advent of the colonial powers in the 19th century. They
organised their empire in vilayets, districts or provinces
controlled by governors who, at times, enjoyed some
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