Double first cousin

Double first cousins arise when two s reproduce with another set of s and the resulting children are related to each other through both s' families. Double first cousins share both sets of s in common and have double the degree of than ordinary s. Genetically they are as related as. 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 s 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.