The rh factor
is determined from both parents.
Each parent contributes one half of the infant's rh type. Each parent has 2 rh genes. So, if each parent (of the child in question) has a "split" rh:(-)/(+), i.e. Half negative and half positive--they are reported as "positive". Yet if each parent contributes the "blank" or rh negative portion to baby then he is (-)/(-) or rh negative!