DEPARTMENT OF GENETICS. 89 



ing in other ways. Misses Bergner and Watt, of this Department, have 

 assisted in hybridization and counting pollen. 



Trisomic Types of Mutants: Primary and Secondary. 



Two of the most interesting developments of the work with Datura are the 

 discovery of the differentiation of the simple trichromosomic types into 

 primary mutants and their respective varieties and the attempt to determine 

 the causes that lie back of such differentiation. 



Dr. Blakeslee reports on this matter as follows : 



"It was stated in our last report that, whereas there should be only 12 

 (2n+ 1) mutants expected if each mutant is caused by the presence of an extra 

 chromosome in a different one of the 12 chromosomal sets, as a matter of 

 fact we have over 20 mutants with a single extra chromosome. It has been 

 possible to arrange the mutants in not over 12 groups. Six of these groups 

 have as yet only a single member, but in the other groups we have a main 

 mutant and one or more varieties. We have sometimes termed these 

 'apostles' and 'acolytes.' The following is some of the evidence for connecting 

 a main mutant with its variety: (1) Similarity in external appearance; (2) 

 similarity in internal anatomy (Dr. Sinnott has successfully grouped the 

 mutants from study of their anatomical structure without knowing how we had 

 classified them from other evidence) ; (3) a study of chromosome size in mutants 

 is being made by Dr. Belling and should give further evidence, positive or 

 negative, on the grouping of mutants; (4) the main mutant and its variety 

 in one group (Poinsettia-wiry) give the same trisomic ratios for a Mendelian 

 factor, apparently carried by the extra chromosome ; (5) so far as our records 

 have been tabulated, it seems to be true that the variety regularly throws a 

 small percentage of its main mutant in the offspring, while the main mutant 

 does not throw its variety except very rarely. 



"As to which in the group is to be considered the main mutant and which 

 the variety is shown by the breeding behavior just mentioned, as well as 

 by the fact that the main mutants occur spontaneously from normals more 

 frequently than their varieties, and especially by the fact that the main 

 mutants are (with only one certain exception) the only (2n+l) types so far 

 thrown by triploid (3n) plants. Since in triploids each chromosomal set is a 

 trisome and the assortment of its members is at random, we should expect, 

 among the viable combinations of chromosomes in the offspring, the 12 main 

 (2n+ 1) mutants. Out of 784 offspring from triploids, 248 have been normals, 

 112 double mutants, and 424 (2n+l) mutants. Of these latter, 51 were 

 Globe, 39 Poinsettia, 38 Cocklebur, 37 Ilex, 18 Echinus, 24 Rolled, 41 Reduced, 

 57 Buckhng, 32 Glossy, 32 Elongate, 50 Microcarpic, and 2 Spinach. None of 

 the varieties appeared in the triploid offspring, with 1 certain exception and 

 2 uncertain determinations. We feel confident that we have in the above 

 list at least 11 of the 12 main mutants. The relatively poor viability of 

 Spinach may account for the small number of this type, but the Spinach group 

 is anomalous in certain respects and shows possible relationships to another 

 group. 



"For some time our efforts have centered upon the relation between the 

 main mutants and their varieties. Our preliminary hypothesis, that the 

 variety might be a main mutant modified by a Mendelian factor, has been 

 shown untenable by an extensive series of breeding-tests. What may be the 

 clue to the situation was obtained this last winter in the breeding behavior of 

 the group of mutants consisting of Cocklebur and Wedge, the latter by various 

 tests shown to be a variety of the former. Previous evidence indicated 



