Hamilton's rule is that an altruistic act shoud be performed whenrtimes the benefit to a recipient is greater than the cost to the donor, whereris a measure of the extent of genetic similarity at the locus determining the action. This rule applies when the action is conditional upon role. Roles X and Y are defined in terms of an asymmetry between the pair of interacting relatives; e.g. X and Y may be older/ younger, stronger/weaker, etc. To obey Hamilton's rule, roles must be assigned randomly with respect to the genotype of the pair of relatives, and a gene must be expressed in one role only; i.e. its expression must be conditional upon role. If gene action is unconditional so that a gene is expressed in both roles, the rule (at least in its simplest form) may not apply.