![Table of Consanguinity showing degrees of relationship](https://upload.wikimedia.org/wikipedia/commons/0/0d/Table_of_Consanguinity_showing_degrees_of_relationship.svg)
Consanguinity ("blood relation", from
Latin
Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through ...
''
consanguinitas'') is the characteristic of having a
kinship
In anthropology, kinship is the web of social relationships that form an important part of the lives of all humans in all societies, although its exact meanings even within this discipline are often debated. Anthropologist Robin Fox says that ...
with another person (being descended from a common ancestor).
Many jurisdictions have laws prohibiting people who are related by blood from
marrying or having sexual relations with each other. The
degree of consanguinity that gives rise to this prohibition varies from place to place. Such rules are also used to determine
heir
Inheritance is the practice of receiving private property, titles, debts, entitlements, privileges, rights, and obligations upon the death of an individual. The rules of inheritance differ among societies and have changed over time. Offic ...
s of an estate according to statutes that govern intestate succession, which also vary from jurisdiction to jurisdiction. In some places and time periods,
cousin marriage
A cousin marriage is a marriage where the spouses are cousins (i.e. people with common grandparents or people who share other fairly recent ancestors). The practice was common in earlier times, and continues to be common in some societies toda ...
is allowed or even encouraged; in others, it is
taboo
A taboo or tabu is a social group's ban, prohibition, or avoidance of something (usually an utterance or behavior) based on the group's sense that it is excessively repulsive, sacred, or allowed only for certain persons.''Encyclopædia Britannica ...
, and considered to be
incest
Incest ( ) is human sexual activity between family members or close relatives. This typically includes sexual activity between people in consanguinity (blood relations), and sometimes those related by affinity ( marriage or stepfamily), ado ...
.
The degree of relative consanguinity can be illustrated with a ''consanguinity table'' in which each level of lineal consanguinity (''
generation
A generation refers to all of the people born and living at about the same time, regarded collectively. It can also be described as, "the average period, generally considered to be about 20–30 years, during which children are born and gr ...
'' or ''
meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
'') appears as a row, and individuals with a collaterally consanguineous relationship share the same row. The Knot System is a numerical notation that describes consanguinity using the
Ahnentafel numbers of shared ancestors.
Legal definitions
Modern secular law
The degree of kinship between two people may give rise to several legal issues. Some laws prohibit
sexual relations between closely-related people, referred to as incestuous. Laws may also bar marriage between closely-related people, which are almost universally prohibited to the second degree of consanguinity. Some jurisdictions forbid marriage between
first cousins, while others do not. Marriage with aunts and uncles (
avunculate marriage
An avunculate marriage is a marriage with a parent's sibling or with one's sibling's child—i.e., between an uncle or aunt and their niece or nephew. Such a marriage may occur between biological (consanguine) relatives or between persons related ...
) is legal in several countries.
Consanguinity is also relevant to inheritance, particularly with regard to
intestate succession
Intestacy is the condition of the estate of a person who dies without having in force a valid will or other binding declaration. Alternatively this may also apply where a will or declaration has been made, but only applies to part of the esta ...
. In general, laws tend to favor inheritance by persons closely related to the deceased. Some jurisdictions ban citizens from service on a jury on the basis of consanguinity as well as affinity with persons involved in the case. In many countries, laws prohibiting
nepotism
Nepotism is an advantage, privilege, or position that is granted to relatives and friends in an occupation or field. These fields may include but are not limited to, business, politics, academia, entertainment, sports, fitness, religion, an ...
ban employment of, or certain kinds of contracts with, the near relations of public officers or employees.
Religious and traditional law
Christianity
Under
Roman civil law
Roman law is the legal system of ancient Rome, including the legal developments spanning over a thousand years of jurisprudence, from the Twelve Tables (c. 449 BC), to the ''Corpus Juris Civilis'' (AD 529) ordered by Eastern Roman emperor Justin ...
, which the early
canon law of the Catholic Church
The canon law of the Catholic Church ("canon law" comes from Latin ') is "how the Church organizes and governs herself". It is the system of laws and ecclesiastical legal principles made and enforced by the hierarchical authorities of the Cathol ...
followed, couples were forbidden to marry if they were within four degrees of consanguinity.
Around the ninth century the church raised the number of prohibited degrees to seven and changed the method by which they were calculated; instead of the former Roman practice of counting each generational link up to the common ancestor and then down again to the proposed spouse as a single degree, the new method computed consanguinity only by counting back the number of generations to the common ancestor.
Intermarriage was now prohibited to anyone more closely related than seventh cousins, which meant that in particular the nobility struggled to find partners to marry, the pool of non-related prospective spouses having become substantially smaller. They had to either defy the church's position or look elsewhere for eligible marriage candidates.
In the
Roman Catholic Church
The Catholic Church, also known as the Roman Catholic Church, is the largest Christian church, with 1.3 billion baptized Catholics worldwide . It is among the world's oldest and largest international institutions, and has played a ...
, unknowingly marrying a closely consanguineous blood relative was grounds for a
declaration of nullity
In the Catholic Church, a declaration of nullity, commonly called an annulment and less commonly a decree of nullity, and by its detractors, a "Catholic divorce", is an ecclesiastical tribunal determination and judgment that a marriage was inval ...
, but during the eleventh and twelfth centuries
dispensations were granted with increasing frequency due to the thousands of persons encompassed in the prohibition at seven degrees and the hardships this posed for finding potential spouses.
In 1215 the
Fourth Lateran Council
The Fourth Council of the Lateran or Lateran IV was convoked by Pope Innocent III in April 1213 and opened at the Lateran Palace in Rome on 11 November 1215. Due to the great length of time between the Council's convocation and meeting, many ...
made what they believed was a necessary change to canon law reducing the number of prohibited degrees of consanguinity from seven back to four, but retaining the later method of calculating degrees.
After 1215, the general rule was that fourth cousins could marry without dispensation, greatly reducing the need for dispensations.
In fourteenth century England, for example, papal dispensations for annulments due to consanguinity (and
affinity) were relatively few.
The ban on marriage to minor degrees of relationship imposed by the Roman Catholic Church was met with heavy criticism in the Croatian society in the 11th century, which led to a schism in the Croatian church.
Among the Christian ''Habesha'' highlanders of
Ethiopia
Ethiopia, , om, Itiyoophiyaa, so, Itoobiya, ti, ኢትዮጵያ, Ítiyop'iya, aa, Itiyoppiya officially the Federal Democratic Republic of Ethiopia, is a landlocked country in the Horn of Africa. It shares borders with Eritrea to the ...
and
Eritrea
Eritrea ( ; ti, ኤርትራ, Ertra, ; ar, إرتريا, ʾIritriyā), officially the State of Eritrea, is a country in the Horn of Africa region of Eastern Africa, with its capital and largest city at Asmara. It is bordered by Ethiopi ...
(the predominantly orthodox Christian
Amhara and
Tigray-
Tigrinya), it is a tradition to be able to recount one's paternal ancestors at least seven generations away starting from early childhood, because "those with a common patrilineal ancestor less than seven generations away are considered 'brother and sister' and may not marry." The rule is less strict on the mother's side, where the limit is about four generations back, but still determined patrilinearly. This rule does not apply to Muslims or other ethnic groups.
Islam
The
Quran
The Quran (, ; Standard Arabic: , Quranic Arabic: , , 'the recitation'), also romanized Qur'an or Koran, is the central religious text of Islam, believed by Muslims to be a revelation from God. It is organized in 114 chapters (pl.: , ...
at 4:22–24 states. "Forbidden to you in marriage are: your mothers, your daughters, your sisters, your father's sisters, your mother's sisters, your brother's daughters, your sister's daughters." Therefore, the list of forbidden marriage partners, as read in the Qur'an, Surah 4:23, does not include first cousins.
Muhammad
Muhammad ( ar, مُحَمَّد; 570 – 8 June 632 Common Era, CE) was an Arab religious, social, and political leader and the founder of Islam. According to Muhammad in Islam, Islamic doctrine, he was a prophet Divine inspiration, di ...
himself married his first cousin
Zaynab bint Jahsh
Zaynab bint Jaḥsh ( ar, زينب بنت جحش; 590–641 CE), was a first cousin and wife of Muhammad and therefore considered by Muslims to be a Mother of the Believers. Abdulmalik ibn Hisham. ''Notes to Ibn Ishaq's "Life of the Prophet"'' ...
.
Financial incentives to discourage consanguineous marriages exist in some countries: mandatory premarital screening for inherited blood disorders has existed in the UAE since 2004 and in Qatar since 2009, whereby couples with positive results will not receive their marriage grant.
[
]
Hinduism
In the ''Manusmriti
The ''Manusmṛiti'' ( sa, मनुस्मृति), also known as the ''Mānava-Dharmaśāstra'' or Laws of Manu, is one of the many legal texts and constitution among the many ' of Hinduism. In ancient India, the sages often wrote their ...
'', blood relation marriage (on the mother's side) is prohibited for 7 generations.
Ayurveda
Ayurveda () is an alternative medicine system with historical roots in the Indian subcontinent. The theory and practice of Ayurveda is pseudoscientific. Ayurveda is heavily practiced in India and Nepal, where around 80% of the population repo ...
states that marriage within the Gotra
In Hindu culture, the term gotra (Sanskrit: गोत्र) is considered to be equivalent to lineage. It broadly refers to people who are descendants in an unbroken male line from a common male ancestor or patriline. Generally, the gotra fo ...
(father's side) is a consanguineous marriage which can lead to many gestational and genetic problems in the fetus. Therefore, it has become a common practice in Hindu households during pre-marriage discussions to ask the couples' Gotra. Couples of the same Gotra are advised not to marry. The advisers of this system say that this practice helps to reduce gestational problems and ensures a healthy progeny.
Genetic definitions
Genetically, consanguinity derives from the reduction in variation due to meiosis
Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately ...
that occurs because of the smaller number of near ancestors. Since all humans share between 99.6% and 99.9% of their genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ge ...
, consanguinity only affects a very small part of the sequence. If two siblings have a child, the child has only two rather than four grandparents. In these circumstances, the probability is increased that the child will inherit two copies of a harmful recessive gene
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
(allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
) (rather than only one, which is less likely to have harmful effects).
Genetic consanguinity is expressed as defined 1922 by Wright with the coefficient of relationship
The coefficient of relationship is a measure of the degree of consanguinity (or biological relationship) between two individuals. The term coefficient of relationship was defined by Sewall Wright in 1922, and was derived from his definition of th ...
''r'', where ''r'' is defined as the fraction of homozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
due to the consanguinity under discussion. Thus, a parent and child pair has a value of ''r''=0.5 (sharing 50% of genes), siblings have a value of ''r''=0.5, a parent's sibling has ''r''=0.25 (25% of genes), and first cousins have ''r''=0.125 (12.5% of genes). These are often expressed in terms of a percentage of shared DNA.
As a working definition, unions contracted between persons biologically related as second cousins or closer (''r'' ≥ 0.03125) are categorized as consanguineous. This arbitrary limit has been chosen because the genetic influence in marriages between couples related to a lesser degree would usually be expected to differ only slightly from that observed in the general population. Globally it is estimated that at least 8.5% of children have consanguineous parents.
In clinical genetics, consanguinity is defined as a union between two individuals who are related as second cousins or closer, with the inbreeding coefficient (F) equal or higher than 0.0156.where (F) represents the proportion of genetic loci at which the child of a consanguineous couple might inherit identical gene copies from both parents.
It is common to identify one's first- and second-degree cousins, and sometimes third-degree cousins. It is seldom possible to identify fourth-degree cousins, since few people can trace their full family tree back more than four generations. (Nor is it considered important, since fourth cousins tend to be genetically no more similar to each other than they are to any other individual from the same region.)
Epidemiology, rates of occurrence
Cultural factors in favor
Reasons favoring consanguinous marriage have been listed as higher compatibility between husband and wife sharing same social relationships, couples stability, enforcing family solidarity, easier financial negotiations and others.[ Consanguinity is a deeply rooted phenomenon in 20% of the world population, mostly in the Middle East, West Asia and North Africa.][ Globally, the most common form of consanguineous union is between first cousins, in which the spouses share of their genes inherited from a common ancestor, and so their progeny are homozygous (or more correctly autozygous) at of all loci (''r'' = 0.0625). Due to variation in geographical and ethnic background and the loci chosen to genotype there is some 2.4% variation expected.
]
Europe
Historically, some European nobles cited a close degree of consanguinity when they required convenient grounds for divorce, especially in contexts where religious doctrine forbade the voluntary dissolution of an unhappy or childless marriage.
Muslim countries
In the Arab world, the practice of marrying relatives is common. According to the Centre for Arabic Genomic Research, between 40% and 54% of UAE nationals' marriages are between family members, up from 39% in the previous generation. Between 21% and 28% of marriages of UAE nationals were between first cousins.[Consanguineous marriage: Should it be discouraged?](_blank)
June 2012, MiddleEastHealthMag.com, retrieved 28 Nov 2018 Consanguineous marriage is much less prevalent in Christian Arabs as they do not practice arranged marriages. Additionally, an indult dispensation is required to marriages contracted between first cousins or closer in Arab Christian denominations in communion with the Roman Catholic Church
The Catholic Church, also known as the Roman Catholic Church, is the largest Christian church, with 1.3 billion baptized Catholics worldwide . It is among the world's oldest and largest international institutions, and has played a ...
, and the Greek Orthodox Church
The term Greek Orthodox Church (Greek: Ἑλληνορθόδοξη Ἐκκλησία, ''Ellinorthódoxi Ekklisía'', ) has two meanings. The broader meaning designates "the entire body of Orthodox (Chalcedonian) Christianity, sometimes also call ...
; there are no similar regulations that apply to first-cousin marriages in the Coptic Orthodox Church.
In Egypt, around 40% of the population marry a cousin. A 1992 survey in Jordan found that 32% were married to a first cousin; a further 17.3% were married to more distant relatives. 67% of marriages in Saudi Arabia
Saudi Arabia, officially the Kingdom of Saudi Arabia (KSA), is a country in Western Asia. It covers the bulk of the Arabian Peninsula, and has a land area of about , making it the fifth-largest country in Asia, the second-largest in the A ...
are between close relatives as are 54% of all marriages in Kuwait
Kuwait (; ar, الكويت ', or ), officially the State of Kuwait ( ar, دولة الكويت '), is a country in Western Asia. It is situated in the northern edge of Eastern Arabia at the tip of the Persian Gulf, bordering Iraq to the nort ...
, whereas 18% of all Lebanese were between blood relatives. The incidence of consanguinity was 54.3% among Kuwaiti natives and higher among Bedouins.
It has been estimated that 55% of marriages between Pakistani Muslim immigrants in the United Kingdom are between first cousins, where preferential patrilateral parallel cousin marriage
A cousin marriage is a marriage where the spouses are cousins (i.e. people with common grandparents or people who share other fairly recent ancestors). The practice was common in earlier times, and continues to be common in some societies toda ...
, i.e. a boy marrying the daughter of his father's brother is favored.
Double first cousin
Most generally, in the lineal kinship system used in the English-speaking world, a cousin is a type of familial relationship in which two relatives are two or more familial generations away from their most recent common ancestor. Commonly, ...
s are descended from two pairs of siblings, and have the same genetic similarity as half-siblings. In unions between double first cousins the highest inbreeding
Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and o ...
coefficients are reached, with an (F) of 0.125, for example in among Arabs and uncle-niece marriages in South India.[
]
Genetic disorders
The phenomenon of inbreeding increases the level of homozygotes for autosomal genetic disorders and generally leads to a decreased biological fitness
Fitness (often denoted w or ω in population genetics models) is the quantitative representation of individual reproductive success. It is also equal to the average contribution to the gene pool of the next generation, made by the same individua ...
of a population known as inbreeding depression
Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding (the breeding of related individuals). Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. In ...
, a major objective in clinical studies. While the risks of inbreeding
Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and o ...
are well-known, informing minority group
The term 'minority group' has different usages depending on the context. According to its common usage, a minority group can simply be understood in terms of demographic sizes within a population: i.e. a group in society with the least number o ...
families with a tradition
A tradition is a belief or behavior (folk custom) passed down within a group or society with symbolic meaning or special significance with origins in the past. A component of cultural expressions and folklore, common examples include holidays or ...
of endogamy
Endogamy is the practice of marrying within a specific social group, religious denomination, caste, or ethnic group, rejecting those from others as unsuitable for marriage or other close personal relationships.
Endogamy is common in many cultu ...
and changing their behavior is a challenging task for genetic counseling
Genetic counseling is the process of investigating individuals and families affected by or at risk of genetic disorders to help them understand and adapt to the medical, psychological and familial implications of genetic contributions to disease; t ...
in the health care system. The offspring of consanguineous relationships are at greater risk of certain genetic disorders. Autosomal recessive
In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...
disorders occur in individuals who are homozygous
Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism.
Mo ...
for a particular recessive gene mutation
In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, mi ...
.[William J Marshall, Ph. D.; S K Bangert, ''Clinical biochemistry : metabolic and clinical aspects'' (Edinburgh; New York: Churchill Livingstone/Elsevier, 2008), p. 920] This means that they carry two copies (allele
An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution.
::"The chro ...
s) of the same gene. Except in certain rare circumstances (new mutations or uniparental disomy
Uniparental disomy (UPD) occurs when a person receives two copies of a chromosome, or of part of a chromosome, from one parent and no copy from the other parent. UPD can be the result of heterodisomy, in which a pair of non-identical chromosomes ar ...
) both parents of an individual with such a disorder will be carriers of the gene. Such carriers are not affected and will not display any signs that they are carriers, and so may be unaware that they carry the mutated gene. As relatives share a proportion of their genes, it is much more likely that related parents will be carriers of an autosomal recessive gene, and therefore their children are at a higher risk of an autosomal recessive disorder.[Benjamin Pierce, ''Genetics: A Conceptual Approach'' (New York: W.H. Freeman, 2012), p. 138] The extent to which the risk increases depends on the degree of genetic relationship between the parents; so the risk is greater in mating relationships where the parents are close relatives, but for relationships between more distant relatives, such as second cousins, the risk is lower (although still greater than the general population).
Consanguinity in a population increases its susceptibility to infectious pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
s such as tuberculosis
Tuberculosis (TB) is an infectious disease usually caused by '' Mycobacterium tuberculosis'' (MTB) bacteria. Tuberculosis generally affects the lungs, but it can also affect other parts of the body. Most infections show no symptoms, in ...
and hepatitis
Hepatitis is inflammation of the liver tissue. Some people or animals with hepatitis have no symptoms, whereas others develop yellow discoloration of the skin and whites of the eyes (jaundice), poor appetite, vomiting, tiredness, abdominal pa ...
but may decrease its susceptibility to malaria and other pathogens.
See also
References
External links
* Alan Bittles
Consanguineous marriages, pearls and perils: Geneva International Consanguinity Workshop Report.
May 2010
*Province of Pennsylvania
statute prohibiting adultery and fornication
(1705), with table of consanguinity, extracted fro
''Smith's Laws''
* Kalmes, Robert and Jean-Loup Huret
* Burtsell, Richard L. ttp://www.newadvent.org/cathen/04264a.htm "Consanguinity (in Canon Law)."''The Catholic Encyclopedia
The ''Catholic Encyclopedia: An International Work of Reference on the Constitution, Doctrine, Discipline, and History of the Catholic Church'' (also referred to as the ''Old Catholic Encyclopedia'' and the ''Original Catholic Encyclopedia'') i ...
''.
Canon Law and Consanguinity
* Rehder C.W. et al
{{Authority control
Kinship and descent
Incest
Medical genetics
Human genetics
Catholic matrimonial canon law
Endogamy