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37-483: More generally, in the kinship system used in the English-speaking world, a cousin is a type of relationship in which relatives are two or more generations away from their most recent common ancestor. For this definition degrees and removals are used to further specify the relationship. Degree measures how distant the relationship is from the most recent common ancestor(s). If the cousins do not come from

74-566: A category of family members that largely overlaps with the term nuclear family , but without spouses. If the persons are related by blood , the first degree relatives share approximately 50% of their genes. First-degree relatives are a common measure used to diagnose risks for common diseases by analyzing family history. A second-degree relative (SDR) is someone who shares 25% of a person's genes. It includes uncles , aunts , nephews , nieces , grandparents , grandchildren , half-siblings and double-first cousins. Third-degree relatives are

111-411: A coefficient of r = 2 = 0.1%. If follows that the value of r can be given to an accuracy of a few percent if the family tree of both individuals is known for a depth of five generations, and to an accuracy of a tenth of a percent if the known depth is at least ten generations. The contribution to r from common ancestors of 20 generations ago (corresponding to roughly 500 years in human genealogy, or

148-440: A common ancestor to a common ancestor's ancestor), and L(p) is the length of the path p . To give an (artificial) example: Assuming that two individuals share the same 32 ancestors of n = 5 generations ago, but do not have any common ancestors at four or fewer generations ago, their coefficient of relationship would be Individuals for which the same situation applies for their 1024 ancestors of ten generations ago would have

185-536: A common ancestor, passing through no individual which is not a common ancestor more than once. A path coefficient between an ancestor A and an offspring O separated by n {\displaystyle n} generations is given as: where f A {\displaystyle f_{A}} and f O {\displaystyle f_{O}} are the coefficients of inbreeding for A and O, respectively. The coefficient of relationship r B C {\displaystyle r_{BC}}

222-427: A scenario where two monozygotic (identical) twins have children with another pair of monozygotic twins, the resulting double cousins would test as genetically similar as siblings. Couples that are closely related have an increased chance of sharing genes, including mutations that occurred in their family tree. If the mutation is a recessive trait , it will not reveal itself unless both father and mother share it. Due to

259-536: A segment of the extended family and includes first cousins, great-grandparents and great-grandchildren. Third-degree relatives are generally defined by the expected amount of genetic overlap that exists between two people, with the third-degree relatives of an individual sharing approximately 12.5% of their genes. The category includes great-grandparents, great-grandchildren, granduncles, grandaunts, grandnephews, grandnieces, first cousins, half-uncles, half-aunts, half-nieces and half-nephews. Most incest laws concern

296-509: A tool to strengthen the family, conserve its wealth, protect its cultural heritage, and retain the power structure of the family and its place in the community. Some groups encourage cousin marriage while others attach a strong social stigma to it. In some regions in the Middle East , more than half of all marriages are between first or second cousins (in some of the countries in this region, this may exceed 70%). Just outside this region, it

333-458: A type of cousin relationship if they share a common ancestor, and are separated from their most recent common ancestor by two or more generations. This means neither person is an ancestor of the other, they do not share a parent (are not siblings ), and neither is a sibling of the other's parent (are not the other's aunt / uncle nor niece/nephew ). In the English system the cousin relationship

370-415: A value of 1, expressed as a percentage, and approaches a value of 0 for individuals with arbitrarily remote common ancestors. The coefficient of relationship ( r {\displaystyle r} ) between two individuals B and C is obtained by a summation of coefficients calculated for every line by which they are connected to their common ancestors . Each such line connects the two individuals via

407-433: Is David's maternal cousin and David is Emma's paternal cousin. On the other hand, parallel and cross cousins are reciprocal relationships. Parallel cousins are descended from same-sex siblings. A parallel first cousin relationship exists when both the subject and relative are maternal cousins, or both are paternal cousins. Cross cousins are descendants from opposite-sex siblings. A cross first cousin relationship exists when

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444-416: Is further detailed by the concepts of degree and removal . The degree is the number of generations subsequent to the common ancestor before a parent of one of the cousins is found. This means the degree is the separation of the cousin from the common ancestor less one. Also, if the cousins are not separated from the common ancestor by the same number of generations, the cousin with the smallest separation

481-407: Is now obtained by summing over all path coefficients: By assuming that the pedigree can be traced back to a sufficiently remote population of perfectly random-bred stock ( f A = 0 for all A in the sum) the definition of r may be simplified to where p enumerates all paths connecting B and C with unique common ancestors (i.e. all paths terminate at a common ancestor and may not pass through

518-520: Is often legal but infrequent. Many cultures have encouraged specifically cross-cousin marriages. In other places, it is legally prohibited and culturally equivalent to incest . Supporters of cousin marriage often view the prohibition as discrimination , while opponents claim potential immorality . Lineal kinship Eskimo kinship is a category of kinship used to define family organization in anthropology . Identified by Lewis H. Morgan in his 1871 work Systems of Consanguinity and Affinity of

555-460: Is one-eighth. For each additional removal of the cousin relationship, consanguinity is reduced by half, as the generations of separation increase by one. For each additional degree of the cousin relationship, consanguinity is reduced by a quarter as the generations of separation increase by one on both sides. Half cousins have half the consanguinity of ordinary cousins as they have half the common ancestors (i.e. one vs two). Double cousins have twice

592-512: Is relatively common among the world's kinship systems, at about 10% of the world's societies. It is now common in most Western societies (such as those of Europe or the Americas). In addition, it is found among a small number of food-foraging peoples such as the ǃKung tribe of Africa and the Inuit (Inuit- Yupik ) for whom it is named. The system is widely used in non-unilineal societies, where

629-423: Is represented as Φ ij . The kinship coefficient between a non-inbred individual and itself, Φ ii , is equal to 1/2. This is due to the fact that humans are diploid , meaning the only way for the randomly chosen alleles to be identical by descent is if the same allele is chosen twice (probability 1/2). Similarly, the relationship between a parent and a child is found by the chance that the randomly picked allele in

666-477: Is the cousin of one's spouse or the spouse of one's cousin. Consanguinity is a measure of how closely individuals are related to each other. It is measured by the coefficient of relationship . Below, when discussing the coefficient of relationship, we assume the subject and the relative are related only through the kinship term. A coefficient of one represents the relationship one has with oneself. Consanguinity decreases by half for every generation of separation from

703-493: Is used to determine the degree. The removal is the difference between the number of generations from each cousin to the common ancestor. Two people can be removed but be around the same age due to differences in birth dates of parents, children, and other relevant ancestors. For example, David and George are on opposing sides of the family tree and are first cousins (David is of the lowest order: one) with one removal (the cousins are of order one and two respectively; therefore,

740-559: The most recent common ancestor , as there are two parents for each child. When there is more than one common ancestor, the consanguinity between each ancestor is added together to get the final result. Between first cousins, there are two shared ancestors each with four generations of separation, up and down the family tree: ( 1 2 ) 4 + ( 1 2 ) 4 {\displaystyle \left({\tfrac {1}{2}}\right)^{4}+\left({\tfrac {1}{2}}\right)^{4}} ; their consanguinity

777-450: The probability that a pair of randomly sampled homologous alleles are identical by descent . More simply, it is the probability that an allele selected randomly from an individual, i, and an allele selected at the same autosomal locus from another individual, j, are identical and from the same ancestor. The coefficient of relatedness is equal to twice the kinship coefficient. The kinship coefficient between two individuals, i and j,

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814-567: The Human Family , the Eskimo system was one of six major kinship systems (Eskimo, Hawaiian , Iroquois , Crow , Omaha , and Sudanese ). The system of English-language kinship terms falls into the Eskimo type. The joint family system places no distinction between patrilineal and matrilineal relatives; instead, it focuses on differences in kinship distance (the closer the relative is,

851-441: The consanguinity of ordinary cousins as they have twice the number of common ancestors (i.e. four vs two). Double first cousins share the same consanguinity as half-siblings. Likewise, double half cousins share the same consanguinity as first cousins as they both have two common ancestors. If there are half-siblings on one side and full siblings on the other, they would have three-halves the consanguinity of ordinary first cousins. In

888-432: The contribution from common descent from a medieval population) falls below one part-per-million . The coefficient of relationship is sometimes used to express degrees of kinship in numeric terms in human genealogy . In human relationships, the value of the coefficient of relationship is usually calculated based on the knowledge of a full family tree extending to a comparatively small number of generations, perhaps of

925-403: The difference in generations to the common ancestor is one). A maternal cousin is a cousin that is related to the mother's side of the family, while a paternal cousin is a cousin that is related to the father's side of the family. This relationship is not necessarily reciprocal, as the maternal cousin of one person could be the paternal cousin of the other. In the example Basic family tree , Emma

962-492: The direct line of descent). The Eskimo system is defined by its "cognatic" or "bilateral" emphasis - no distinction is made between patrilineal and matrilineal relatives. Parental siblings are distinguished only by their sex (aunt, uncle). All children of these individuals are lumped together regardless of sex (cousins). Unlike the Hawaiian system , Ego's parents are clearly distinguished from their siblings. The Eskimo system

999-639: The dominant relatives are the immediate family. In most Western societies, the nuclear family represents an independent social and economic group, which has caused the emphasis on the immediate kinship. The tendency of families in Western societies to live apart also reinforces this. The term Eskimo is considered pejorative in Canada, and has been replaced there by the term Inuit . The former remains in use in Alaska, though less so than in past decades, because

1036-408: The likelihood of immunological incompatibility during pregnancy. Cousin marriage is important in several anthropological theories, which often differentiate between matriarchal and patriarchal parallel and cross cousins. Currently about 10% and historically as high as 80% of all marriages are between first or second cousins. Cousin marriages are often arranged . Anthropologists believe it is used as

1073-410: The more distinctions are made). The system emphasizes the nuclear family , identifying directly only the mother, father, brother, and sister. All other relatives are grouped together into categories. It uses both classificatory and descriptive terms, differentiating between gender, generation, lineal relatives (relatives in the direct line of descent), and collateral relatives (blood relatives not in

1110-464: The order of three or four. As explained above, the value for the coefficient of relationship so calculated is thus a lower bound, with an actual value that may be up to a few percent higher. The value is accurate to within 1% if the full family tree of both individuals is known to a depth of seven generations. A first-degree relative (FDR) is a person's parent (father or mother), sibling (brother or sister) or child (son or daughter). It constitutes

1147-422: The relationships where r = 25% or higher, although many ignore the rare case of double first cousins. Some jurisdictions also prohibit sexual relations or marriage between cousins of various degree, or individuals related only through adoption or affinity . Whether there is any likelihood of conception is generally considered irrelevant. The kinship coefficient is a simple measure of relatedness, defined as

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1184-444: The resulting children would be double half first cousins and would share one grandparent on each side. While there is no agreed upon term, it is possible for cousins to share three grandparents if a pair of half siblings had children with a pair of full siblings. Step-cousins are either stepchildren of an individual's aunt or uncle, nieces and nephews of one's step-parent, or the children of one's parent's step-sibling. A cousin-in-law

1221-485: The risk that the trait is harmful, children of high-consanguinity parents have an increased risk of recessive genetic disorders . See inbreeding for more information. Closely related couples have more children. Couples related with consanguinity equivalent to that of third cousins have the greatest reproductive success . This seems counterintuitive as closely related parents have a higher probability of having offspring that are unfit , yet closer kinship can also decrease

1258-459: The same generation, removal is specified, as removal measures the difference in generations between the two cousins. When the removal is not specified, no removal is assumed. Various governmental entities have established systems for legal use that can precisely specify kinship with common ancestors any number of generations in the past; for example, in medicine and in law, a first cousin is a type of third-degree relative . People are related with

1295-772: The subject and the relative are maternal cousin and paternal cousin to each other. In the basic family tree example, David and Emma are cross cousins. Double cousins are relatives who are cousins from two different branches of the family tree. This occurs when siblings , respectively, reproduce with different siblings from another family. This may also be referred to as "cousins on both sides". The resulting children are related to each other through both their parents and are thus doubly related. Double first cousins share both sets of grandparents . Half cousins are descended from half siblings and would share one grandparent. The children of two half siblings are first half cousins. If half siblings have children with another pair of half siblings,

1332-407: The term includes both Inuit and non-Inuit Native Alaskans . In Canada, the term Inuit kinship is therefore widely used instead of Eskimo kinship . 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

1369-410: 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 general, the higher the level of inbreeding the closer the coefficient of relationship between the parents approaches

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