Double first cousins arise when two siblings reproduce with another set of siblings and 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. 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. The coefficent of relationship between double first cousins is 25%.
When identical twins reproduce with identical twins the resulting children are genetically indistinguishable from full siblings. When identical twins reproduce with siblings the resulting children are more related than half-siblings but less related than full siblings.
Children of double first cousins are double second cousins.
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double half first cousins
Double half cousins arise when two half siblings reproduce with another set of half siblings. Double Half Cousins would share 2 grandparents. Unlike regular first cousins double half cousins would not be descended from a shared coupling of grandparents and would instead share a grandparent on both sides of their family. The coefficent of relationship between double half cousins is 12.5% which is the same as first cousins.
irregular double first cousins
When two siblings reproduce with a set of half siblings the resulting double cousins would be irregular as they would be full cousins through one set of parents but half cousins through the other set of parents. Irregular Double Cousins would share 3 grandparents. The coefficent of relationship between irregular double cousins is 18.75%.