About 50 severe recessive autosomal or X-linked diseases can now be detected in utero by biochemical assays of cultured amniotic fluid cells (for reviews, see: Milunsky et al., 1970; Milunsky & Littlefield, 1972; Burton et al.,1974). In autosomal recessive diseases the diagnosis of a first affected child usually is the first indication that both parents are carriers of the same gene mutation. The recurrence risk is 25%. Prenatal diagnosis in future pregnancies of these parents will only be reliable if cultured cells of the affected child are a-vailable. The prenatal diagnosis will be based on a comparison of the bio­chemical assays on amniotic fluid cells from the pregnancy at risk, control cells from a normal pregnancy cultured under identical conditions and fibro- blasts from the patient with the metabolic defect.

The birth of a first affected child can be prevented only in those circumstan­ces where the carriership of the parents has been identified before their first pregnancy. This is sometimes possible in X-linked diseases; for autosomal dis­orders this is generally not the case. Large-scale carrier testing for these di­seases will only be useful in populations with a high frequency for certain gene mutations. The only example of such a program is the carrier-testing for Tay-Sachs' disease in certain Ashkenazi-Jew population groups in the United States, where prenatal diagnosis can be offered to couples identified as hete-rozygotes (Kaback et al., 1974).