Consanguinity

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The number next to each box in the Table of Consanguinity indicates the degree of relationship relative to the given person. Table of Consanguinity showing degrees of relationship.svg
The number next to each box in the Table of Consanguinity indicates the degree of relationship relative to the given person.

Consanguinity ("blood relation", from Latin consanguinitas ) is the property of being from the same kinship as another person. In that aspect, consanguinity is the quality of being descended from the same ancestor as another person.

Contents

The laws of many jurisdictions set out the degree of consanguinity in relation to prohibited sexual relations and marriage parties. Such rules are also used to determine heirs of an estate according to statutes that govern intestate succession, which vary from jurisdiction to jurisdiction. In some places and times, cousin marriage is approved and expected; in others, it is taboo as incest.

The degree of relative consanguinity can be illustrated with a consanguinity table in which each level of lineal consanguinity ( generation or meiosis ) appears as a row, and individuals with a collaterally consanguineous relationship share the same row. [1] The Knot System is a numerical notation that defines consanguinity. [2]

Consanguinity of the kings of France as shown in Arbor genealogiae regum Francorum (Bernard Gui, early 14th century). Bernard Gui BNF lat4975.jpg
Consanguinity of the kings of France as shown in Arbor genealogiae regum Francorum (Bernard Gui, early 14th century).

Modern secular law

The degree of kinship between two people may give raise 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) is legal in several countries.

Consanguinity is also relevant to inheritance, particularly with regard to intestate succession. 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 and also affinity with persons involved in the case. [3]

In many countries, laws prohibiting nepotism ban employment of, or certain kinds of contracts with, the near relations of public officers or employees.

Religious and traditional law

Christianity

Scale of justice, canon law.svg
Part of a series on the
Canon law of the
Catholic Church
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Under Roman civil law, which early canon law of the Catholic Church followed, couples were forbidden to marry if they were within four degrees of consanguinity. [4] In the ninth century the church raised the number of prohibited degrees to seven and changed the method by which they were calculated. [4] This meant that the nobility struggled to find partners to marry, as the pool of non-related prospective spouses became smaller. They had to either defy the church's position or look elsewhere for eligible marriage candidates. [4] In 1215 the Fourth Lateran Council made what they believed was a necessary change to canon law reducing the number of prohibited degrees of consanguinity from seven back to four. [5] [6] The method of calculating prohibited degrees was changed also: Instead of the former practice of counting up to the common ancestor then down to the proposed spouse, the new law computed consanguinity by counting back to the common ancestor. [4] In the Roman Catholic Church, unknowingly marrying a closely consanguineous blood relative was grounds for a declaration of nullity, 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. [7]

After 1215, the general rule was that while fourth cousins could marry without dispensation, generally the need for dispensations was greatly reduced. [7] In fourteenth century England, for example, papal dispensations for annulments due to consanguinity (and affinity) were relatively few. [8]

The connotations of degree of consanguinity varies by context, though most cultures define a degree of consanguinity within which sexual interrelationships are regarded as incestuous or the "prohibited degree of kinship".

Among the Christian Habesha highlanders of Ethiopia and Eritrea (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. [9]

Islam

The Quran 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." [10] Therefore, the list of forbidden marriage partners, as read in the Qur'an, Surah 4:23, does not include first cousins. [11] Muhammad himself married his first cousin Zaynab bint Jahsh. [12] [ better source needed ]

Financial incentives to discourage consangineous marriages exist in some countries: mandatory premarital screening for inherited blood disorders exist in the UAE since 2004, Qatar in 2009, where couples with positive results will not receive their marriage grant. [13]

Hinduism

In the Manusmriti , blood relation marriage (on the mother's side) is prohibited for 7 generations.[ citation needed ]

Ayurveda states that marriage within the Gotra (father's side) is a consanguineous marriage which can lead to many gestational and genetic problems in the fetus. So 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 in reducing gestational problems and ensures a healthy progeny.[ citation needed ]

Genetic definitions

Average DNA shared between relatives [14]
RelationshipAverage DNA
shared %
Identical twin100%
Parent / child50%
Full sibling50%
Fraternal twin50%
Grandparent / grandchild25%
Aunt / uncle / niece / nephew25%
Half-sibling/double first cousin25%
Great-grandparent / great-grandchild12.5%
Great-aunt / great-uncle / great-niece / great-nephew12.5%
First cousin12.5%
First cousin once removed/double 2nd cousin6.25%
2nd cousin / first cousin twice removed3.125%
2nd cousin once removed1.5625%
3rd Cousin0.78125%
4th Cousin0.20%
5th Cousin0.05%
6th Cousin0.01%
A simplistic depiction of genetic relatedness after n generations as a 2 progression. Gene-distribution.png
A simplistic depiction of genetic relatedness after n generations as a 2 progression.
Diagram of common family relationships, where the area of each colored circle is scaled according to the coefficient of relatedness. All relatives of the same relatedness are included together in one of the gray ellipses. Legal degrees of relationship can be found by counting the number of solid-line connections between the self and a relative. Coefficient of relatedness.png
Diagram of common family relationships, where the area of each colored circle is scaled according to the coefficient of relatedness. All relatives of the same relatedness are included together in one of the gray ellipses. Legal degrees of relationship can be found by counting the number of solid-line connections between the self and a relative.

Genetically, consanguinity derives from the reduction in variation due to meiosis that occurs because of the smaller number of near ancestors. Since all humans share between 99.6% and 99.9% of their genome, [15] consanguinity only affects a very small part of the sequence. If two siblings have a child, the child only has two rather than four grandparents. In these circumstances the probability that the child inherits two copies of a harmful recessive gene (allele) rather than one which would not have immediate effects is much increased.

Genetic consanguinity is expressed as defined 1922 by Wright [16] with the coefficient of relationship r, where r is defined as the fraction of homozygous 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. [17]

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. [18]

It is common to distinguish first-degree cousins, second-degree cousins, and often also third-degree cousins. Since comparatively few people can trace their full family tree for more than four generations, the identity of fourth-degree cousins often cannot be established. Also, at a genetic level, half-fourth cousins typically do not exhibit greater genetic similarity with one another than with any other individual from the same population. [19]

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. [18] :187 Consanguinity is a deeply rooted phenomenon in 20% of the world population mostly the Middle East, West Asia and North Africa. [18] Globally, the most common form of consanguineous union is between first cousins, in which the spouses share 18 of their genes inherited from a common ancestor, and so their progeny are homozygous (or more correctly autozygous) at 116 of all loci (r = 0.0625). [20] Due to variation in geographical and ethnic background and the loci chosen to genotype there is some 2.4% variation expected. [21]

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. [22]

Muslim countries

In the Arab world today[ when? ] 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. [13] [23] Consanguineous marriage is much less prevalent in Christian Arabs as they do not practice arranged marriages. [24] [25] [26] [27] 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, and the Greek Orthodox Church; there are no similar regulations that apply to first-cousin marriages in the Coptic Orthodox Church. [27]

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. [28] 67% of marriages in Saudi Arabia are between close relatives as are 54% of all marriages in Kuwait, whereas 18% of all Lebanese were between blood relatives. The incidence of consanguinity was 54.3% among Kuwaiti natives and higher among Bedouins. [29]

It has been estimated that 55% of marriages between Pakistani Muslim immigrants in the United Kingdom are between first cousins, [30] where preferential patrilateral parallel cousin marriage, i.e. a boy marrying the daughter of his father's brother is favored.

Double first cousins 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 coefficients are reached, with an(F) of 0.125, for example in among Arabs and uncle-niece marriages in South India. [18]

Genetic disorders

The phenomenon of inbreeding increases the level of homozygotes for autosomal genetic disorders and generally leads to a decreased biological fitness of a population known as inbreeding depression, a major objective in clinical studies. [31] While the risks of inbreeding are well-known, informing minority group families with a tradition of endogamy and changing their behavior is a challenging task for genetic counseling in the health care system. [32] The offspring of consanguineous relationships are at greater risk of certain genetic disorders. Autosomal recessive disorders occur in individuals who are homozygous for a particular recessive gene mutation. [33] This means that they carry two copies (alleles) of the same gene. [33] Except in certain rare circumstances (new mutations or uniparental disomy) both parents of an individual with such a disorder will be carriers of the gene. [33] 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. [34] 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). [35]

Consanguinity in a population increases its susceptibility to infectious pathogens such as tuberculosis and hepatitis. [36]

See also

Related Research Articles

Incest is human sexual activity between family members or close relatives. This typically includes sexual activity between people in consanguinity, and sometimes those related by affinity, adoption, clan, or lineage.

Inbreeding Production of offspring from the mating of individuals of a breed who are more closely related than the average members of the breed.

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 other consequences that may arise from expression of deleterious or recessive traits resulting from incestuous sexual relationships and consanguinity.

An incest taboo is any cultural rule or norm that prohibits sexual relations between certain members of the same family, mainly between individuals related by blood. All human cultures have norms that exclude certain close relatives from those considered suitable or permissible sexual or marriage partners, making such relationships taboo. However, different norms exist among cultures as to which blood relations are permissible as sexual partners and which are not. Sexual relations between related persons which are subject to the taboo are called incestuous relationships.

Exogamy is the social norm of marrying outside one's social group. The group defines the scope and extent of exogamy, and the rules and enforcement mechanisms that ensure its continuity. One form of exogamy is dual exogamy, in which two groups engage in continual wife exchange.

Kinship Human relationship term; web of social relationships that form an important part of the lives of most humans in most societies; form of social connection

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 states that "the study of kinship is the study of what man does with these basic facts of life – mating, gestation, parenthood, socialization, siblingship etc." Human society is unique, he argues, in that we are "working with the same raw material as exists in the animal world, but [we] can conceptualize and categorize it to serve social ends." These social ends include the socialization of children and the formation of basic economic, political and religious groups.

Endogamy is the practice of marrying within a specific social group, caste, or ethnic group, rejecting those from others as unsuitable for marriage or other close personal relationships.

The coefficient of relationship is a measure of the degree of consanguinity between two individuals. The term coefficient of relationship was defined by Sewall Wright in 1922, and was derived from his definition of the coefficient of inbreeding of 1921. The measure is most commonly used in genetics and genealogy. A coefficient of inbreeding can be calculated for an individual, and is typically one-half the coefficient of relationship between the parents.

In genealogy, pedigree collapse describes how reproduction between two individuals who share an ancestor causes the number of distinct ancestors in the family tree of their offspring to be smaller than it could otherwise be. Robert C. Gunderson coined the term; synonyms include implex and the German Ahnenschwund.

In Catholic canon law, affinity is an impediment to marriage of a couple due to the relationship which either party has as a result of a kinship relationship created by another marriage or as a result of extramarital intercourse. The relationships that give rise to the impediment have varied over time. Marriages and sexual relations between people in an affinity relationship are regarded as incestuous.

Commonly "cousin" refers to a "first cousin", a relative whose most recent common ancestor with the subject is a grandparent. More 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.

A cousin couple is a pair of cousins who are involved in a romantic or sexual relationship.

Inbreeding depression is the reduced biological fitness in a given population as a result of inbreeding, or breeding of related individuals. Population biological fitness refers to an organism's ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a population bottleneck. In general, the higher the genetic variation or gene pool within a breeding population, the less likely it is to suffer from inbreeding depression.

In law and in cultural anthropology, affinity is the kinship relationship created or that exists between two people as a result of someone's marriage. It is the relationship which each party to a marriage has to the relations of the other partner to the marriage, but it does not cover the marital relationship itself. Laws, traditions and customs relating to affinity vary considerably, sometimes ceasing with the death of one of the marriage partners through whom affinity is traced, and sometimes with the divorce of the marriage partners. In addition to kinship by marriage, "affinity" can sometimes also include kinship by adoption or a step relationship.

Cousin marriage Marriage between those with common grandparents or other recent ancestors

A cousin marriage is a marriage where the partners are cousins. The practice was common in earlier times, and continues to be common in some societies today, though in some jurisdictions such marriages are prohibited. Worldwide, more than 10% of marriages are between first or second cousins. Cousin marriage is an important topic in anthropology and alliance theory.

In law, a prohibited degree of kinship refers to a degree of consanguinity and sometimes affinity between persons that results in certain actions between them being illegal. Two major examples of prohibited degrees are found in incest and nepotism. Incest refers to sexual relations and marriage between closely related individuals; nepotism is the preference of blood-relations in the distribution of a rank or office.

Laws regarding incest vary considerably between jurisdictions, and depend on the type of sexual activity and the nature of the family relationship of the parties involved, as well as the age and sex of the parties. Besides legal prohibitions, at least some forms of incest are also socially taboo or frowned upon in most cultures around the world.

The Research Centre for DNA Diagnostics (RCDD) is a medical research facility at Karnatak University, Dharwar, Karnataka, India. The primary objective of the centre is to identify hereditary disorders prevalent in north Karnataka and to conduct advanced research.

There are several autosomal recessive genetic disorders that are more common in ethnically Jewish populations, particularly Ashkenazi Jews, than the population as a whole. This is due to population bottlenecks that occurred relatively recently in the past as well as a practice of consanguineous marriage. These two phenomena lead to a decrease in genetic diversity and a higher likelihood that two parents will carry a mutation in the same gene and pass on both mutations to a child.

Cousin marriage is allowed and often encouraged throughout the Middle East. The bint 'amm marriage, or marriage with one's father's brother's daughter is especially common, especially in tribal and traditional communities. Anthropologists have debated the significance of the practice; some view it as the defining feature of the Middle Eastern kinship system while others note that overall rates of cousin marriage have varied sharply between different Middle Eastern communities. There is very little numerical evidence of rates of cousin marriage in the past.

Consanguine marriage is marriage between individuals who are closely related. Though it may involve incest, it implies more than the sexual nature of incest. In a clinical sense, marriage between two family members who are second cousins or closer qualifies as consanguineous marriage. This is based on the gene copies their offspring may receive. Though these unions are still prevalent in some communities, as seen across the Greater Middle East region, many other populations have seen a great decline in intra-family marriages.

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