Cousin

Basic definitions

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 is used to determine the degree.

Additional terms

• The terms full cousin and cousin-german are used to specify a first cousin with no removals.
• The terms cousin-uncle/aunt and cousin-niece/nephew are sometimes used to describe the direction of the removal of the relationship, especially in Mennonite, For additional removals, grand/great are applied to these relationships. Thus, David is a cousin-granduncle from Ingeborg's perspective, or a cousin twice removed that comes from an older generation, also known as a cousin twice removed up. Similarly, Ingeborg is a cousin-grandniece, or a female first cousin twice removed down, to David.
• The term grandcousin is sometimes used for the grandchild of a first cousin, or the first cousin of a grandparent: a first cousin twice removed.
• The term kissing cousin is sometimes used for a distant relative who is familiar enough to be greeted with a kiss.

Gender-based distinctions

Main article: Parallel and cross cousins

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 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 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.

Multiplicities

Double 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, 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.

Cousins-in-law and non-blood relations

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 is the cousin of one's spouse, the spouse of one's cousin, or the spouse of one's spouse's cousin.

Consanguinity

Main article: Consanguinity

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 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: \left(\tfrac{1}{2}\right)^4 + \left(\tfrac{1}{2}\right)^4; their consanguinity 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 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 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.

Cousin marriage

Main article: Cousin marriage

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.{{cite tech report

Reproduction

Main article: Inbreeding

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 the risk that the trait is harmful, children of high-consanguinity parents have an increased risk of recessive genetic disorders.

Research has indicated that related couples have more children: a study of Icelandic couples concluded that those 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 the likelihood of immunological incompatibility during pregnancy.

The risks of inbreeding extend beyond the immediate descendants of those involved. Sustained cousin marriage within a population group over generations eventually leads to inbreeding at the population level and an increase in genetic disorders that affects the entire population, not just those in cousin marriages. Increased genetic literacy within a population may lead to a reduction in cousin marriage.

References

References

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9. (July 2020}} families. Adding uncle/aunt refers to an older generation while niece/nephew refers to younger ones. Another approach is to use the terms ''up'' and ''down'' to indicate direction of removal. For example, in the ''Basic family tree'' above, George would be a cousin-nephew of David, or a first cousin once removed down. David would be George's cousin-uncle, or cousin once-removed up.{{Cite web). "Rosentreter Family Tree {{!}} Family Relationship Terms and Definitions".
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