2. Directed segregation.
A derived strain from a complex translocation involving chromosomes 5 and 3 has the following constitution: Nine normal bivalents, including chromosome 5, and a chain of three consisting of a normal chromosome 3, a short arm, and a long arm of chromosome 3. When this chain of three is present in plants with a certain genetic background, the orientation of the chain on the metaphase I spindle is approximately random, i.e., orientation of the chain leading to alternate segregation of the three members and giving euploid combinations occurs in 50 per cent of the P.M.C., while a linear orientation leading to aneuploid gametes occurs in 50 per cent of the P.M.C. In other strains, differing in genetic modifiers from the above, the orientation of the chain is such that in about 95 per cent of the cells the normal chromosome 3 passes to one pole while the other two members of the chain pass together to the other pole. Here we apparently have a case of genic control of orientation, and hence segregation. This finding is of interest in connection with the breeding behavior of Oenothera translocations.