In Studies of Extended Families in Which Some Members Are

In studies of extended families in which some members are affected by a disease and other members are unaffected by it, which of the following must be established in order to identify genetic linkage?

A) Both affected and unaffected members have the marker.
B) Family members affected by the disease do not have the marker.
C) The marker is only present in one family and not in other families.
D) Family members with the disease have the marker; unaffected members do not have the marker.