THE BROKEN CHORD By Michael Dorris (Harper Perennial: $9.95, illustrated) . Michael Dorris won a National Book Critics' Circle Award for this deeply moving story that blends personal experience with scientific research. His investigation of the limited abilities of the Sioux boy he adopted led Dorris to the recently named Fetal Alcohol Syndrome (FAS): Drinking during pregnancy can severely damage the delicate brain of the fetus. The problem has reached epidemic proportions on Indian reservations, where alcoholism is rampant. Dorris admits there are no easy solutions to the questions surrounding FAS. Should pregnant women be jailed to prevent them from drinking? How will the presence of so many alcohol-damaged individuals (in some cases, the syndrome has been at work for two and even three generations) affect the future of the Amerindian tribes? How can the social importance of drinking be curbed on the reservations? Sadly, it is too late to do anything for the children with FAS, whose amniotic fluid reeked of cheap wine at birth.