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.
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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