Manic depressive illness in a founder population

Manic depressive illness (MDI) segregates within a founder population originating from S, a mountain village in Southern Italy. Identity by descent of affected persons cannot be established by direct genealogical methods. A 56 000 persons family reconstruction data base encompasses the whole population of S in the 17th and 18th centuries, and part of the population of S and neighbouring villages in the 19th and 20th centuries. We selected 10 MDI probands who were members of the S population and not evident close relatives of each other. A total of 10 other MDI probands not evidently related to the S population formed a first control group. A second control group was formed with 10 not closely related persons originating within the S population. We determined the founders common ancestors to all the probands of one group but not ancestors to the probands of other groups, and computed the genetic contribution of each founder to each proband. The distance between probands in the S MDI group was calculated with respect to all the founders. In all, 17 founders present in the ascendancy of all individuals of the S MDI group are not present in the ascendancy of the control groups: MDI patients are derived from a subpopulation within S. Two S MDI probands are far from one another in respect of the 'nonspecific' founders, but are very close in respect of the specific founders: a putative MDI trait originated from specific founders of the MDI subpopulation, and hence is identical by descent in the S population.