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Double first cousins arise when siblings of one family reproduce with siblings of another family. The resulting children are related to each other through both parents' families. Double first cousins share both sets of grandparents in common and have double the degree of consanguinity than ordinary first cousins. Genetically, they are as related as half-siblings, sharing 25% of their DNA (a coefficient of relationship of 1/4).[1] Their coefficient of coancestry is 1/8th or 0.125. While double first cousins have the same coefficient of coancestry (1/8) as half-siblings, they do have higher chances of sharing both alleles (1/16 vs 0) and lower chances of sharing one allele (3/8 vs 1/2) with each other than half-siblings.
When identical twins reproduce with a pair of siblings, the resulting children are more related than half-siblings but less related than full siblings (they are genetically equivalent to 3/4 siblings) although they are legally double first cousins.
When identical twins reproduce with the same person, the resulting children are likewise genetically indistinguishable from full siblings, although they are genealogically both half-siblings and first cousins.
When two siblings who are not identical twins reproduce with the same person, the resulting children are likewise more related than half-siblings but less related than full siblings. In animal breeding, most notably race horses, these latter are referred to as 3/4 siblings.
Children of double first cousins are double second cousins to each other. There are, however, several additional types of double second cousins, including the children of two sets of regular first cousins.
Double cousin marriage is prohibited in North Carolina. It is permitted in the other 24 states that permit marriage between first cousins.[2]
See also
References
- ^ Arnold, Jackie Smith, Kinship, Genealogical Publishing Co., 2000
- ^ http://www.ncsl.org/default.aspx?tabid=4266
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