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Consanguinity ("blood relation", from the Latin consanguinitas) is the property of being from the same kinship as another person. In that respect, consanguinity is the quality of being descended from the same ancestor as another person. The laws of many jurisdictions set out degrees of consanguinity in relation to prohibited sexual relations and marriage parties or whether a given person inherits property when a deceased person has not left a will.
The degree of relative consanguinity can be illustrated with a consanguinity table, in which each level of lineal consanguinity (i.e., generation or meiosis) appears as a row, and individuals with a collaterally consanguineous relationship share the same row. The Knot System is a numerical notation that defines consanguinity.
Issues of consanguinity arise in several aspects of the law. It is directly relevant in determining whether a couple can marry. These are linked to a jurisdiction's definition of incest, so that couples in an incestuous relationship will not be permitted to marry. Some United States jurisdictions forbid first-cousins to marry, while others limit the prohibition to any direct ancestors or descendants, as well as siblings, half-siblings, aunts and uncles. However, marriage with aunts and uncles (avunculate marriage) is legal in Argentina, Australia, Austria, Brazil, France, Malaysia, and Russia, among other places in the world.
Several volumes of Smith's Laws, enacted from 1700 through 1829, contain certain public and private laws of the Province and Commonwealth of Pennsylvania. Several laws with a prescribed punishment against adultery, bigamy, incest and fornication and all combinations of those crimes were enacted in 1705.
Consanguinity is also relevant in issues of inheritance. In regard to the law of intestate succession (when a person dies without a will), under the Uniform Probate Code of the United States section 2-103, after a surviving spouse receives his or her share, the descendants (depending on the circumstances this may include children, grandchildren, or great grandchildren, either biological or adopted) receive the remainder of the intestate estate. If there are no children, the decedent's parent(s) receive the remainder of the estate. If there are neither descendants nor parents, the decedent's estate is distributed to descendants of the decedent's parents (again, depending on the circumstances, brothers and sisters, nieces and nephews, grand nieces and nephews and great grand nieces and nephews). If there are no descendants, parents, or descendants of parents, then the deceased's property passes to the decedent's grandparents, or if no grandparents, then the descendants of the grandparents of the decedent (uncles and aunts, first cousins, or first cousins once, twice, or thrice removed, second cousins, third cousins etc.).
Adoption may or may not be considered at law to create such a bond; in most Western societies, adoptive relationships are considered blood relationships for these purposes, but in others, including both Japan and ancient Rome, it was common for a couple with only daughters to adopt a son-in-law, making the marriage one between adoptive siblings.
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. In the ninth century the church raised the number of prohibited degrees to seven and changed the method by which they were calculated. Eventually the nobility became too interrelated to marry as the pool of non-related prospective spouses became smaller. It was either defy the church's position or look elsewhere for eligible marriage candidates. 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. 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. In the Roman Catholic Church, unknowingly marrying a closely consanguineous blood relative was grounds for an annulment, 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. After 1215 the general rule was that while fourth cousins could marry without dispensation, generally the need for dispensations was greatly reduced. In fourteenth century England, for example, papal dispensations for annulments due to consanguinity (and affinity) were relatively few.
The connotations of degree of consanguinity varies by context (e.g., Canon law, Roman law, etc.). Most cultures define a degree of consanguinity within which sexual interrelationships are regarded as incestuous (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.
Ayurveda clearly states that marriage within the Gotra is a consanguineous marriage which can lead to many gestational and genetic problems in the fetus. So it has become a common practice in the 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 definitely helps in reducing the gestational problems and ensures a healthy progeny.
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The percentage of consanguinity between any two individuals decreases fourfold as the most recent common ancestor recedes one generation. Consanguinity, as commonly defined, does not depend on the amount of shared DNA within two people's genome. It rather counts the number of meioses separating two individuals. Because of the effects of pedigree collapse, this does not directly translate into the amount of shared genetic substance.
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.
Double first cousins are descended from two pairs of siblings, and have the same genetic similarity as half-siblings.
Conventionally, genetic consanguinity is expressed as defined by Wright 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, siblings have a value of r=0.5, a parent's sibling has r=0.25, and first cousins have r=0.125.
As a working definition, unions contracted between persons biologically related as second cousins or closer (r ≥ 0.0156) 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, the most common form of consanguineous union contracted is between first cousins, in which the spouses share 1/8 of their genes inherited from a common ancestor, and so their progeny are homozygous (or more correctly autozygous) at 1/16 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. In practice the technical factors involved are the design of the SNP genotyping platform was used (e.g. which SNP array or sequencing method) and which software cut-offs were applied.
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. Conversely, the consanguinity law of succession requires the next monarch to be of the same blood of the previous one; allowing, for example, illegitimate children to inherit.
It is estimated that 55% of marriages between Mirpuri (Kashmiri) Pakistani Muslim immigrants in the United Kingdom are between first cousins, where "preferential patrilateral parallel cousin marriage" (where a boy marries the daughter of his father's brother) is often favored.
The offspring of consanguinous relationships are at greater risk of certain genetic disorders. Autosomal recessive disorders occur in individuals who are homozygous for a particular recessive gene mutation. This means that they carry two copies (alleles) of the same gene. 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. 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. 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).
The low genetic heterozygosity associated with increased consanguinity in a population (identified by microsatellite markers) increases its susceptibility to infectious pathogens such as tuberculosis and hepatitis.
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