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.
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When identical twins reproduce with siblings the
resulting children are more related than half-siblings but less related than
full siblings.
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Children of double first cousins are double second
cousins.
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