When used by a breeding's poster (or in a split, the split partner), one whelp in the litter may have an extra allele added from one of the bred parents, even if the only copy was already inherited. 

This should be used either in the litter image stage (via the description or via a comment) or in the geno design request stage, so long as it is applied by the breeding's poster. Dice or its modified version, 20 Sided Dice, can only be used once per litter, though both Dice and Slice may be used and can be used on different whelps in the litter.