DEPARTMENT OF EXPERIMENTAL EVOLUTION. 135 



dominants, and, in harmony with his earlier discovery of a ratio ap- 

 proximating 1:4:1, have been the results of an extensive test of a 

 family in which the recessives were in excess of expectation, for in this 

 family there proved to be an excess of homozygous dominants also, so 

 that the ratio of DD : DR : RR was approximately 1:1.7:1; (e) that there 

 is a correlation between the date at which the plants begin to bloom 

 and the number of dominant genes which they possess. In harmony 

 with this general result, the heegeri individuals usually begin blooming, 

 on the average, several days later than the bursa-pastoris individuals 

 of the same family. 



In tracing the geographic distribution of the duplicate capsule- 

 determiners, the most important step has been the testing of 7 indi- 

 viduals growing in nature at Landau, Germany, where the original 

 B. heegeri was discovered. Of these 7 individuals, 6 had both of the 

 duplicate determiners, C and D, and the other had only a single deter- 

 miner for the triangular capsule, and produced an F2 ratio of about 4: 1. 

 This prevalence of duplicate determiners at Landau was a surprise to 

 Dr. Shull, as he had thought it probable that a single determiner for the 

 triangular capsule would be found generally in the bursa-pastoris from 

 that place. The F2 families from crosses between B. heegeri and a 

 biotype of B. bursa-pastoris from Tucson, Arizona, gave ratios approach- 

 ing 10:1. As this is directly between the expected ratios 3:1 and 

 15:1, only the Fo will demonstrate with certainty to which of these 

 ratios the 10: 1 should be referred. In continuation of this geographic 

 stud}^, crosses have been made between B. heegeri and a number of 

 biotypes of Bursa from various parts of Europe, Asia, and America, 

 and seeds have been secured from as many new regions as possible. 

 Cultures of B. grandiflora, B. viguieri, and B. bursa-pastoris apetala 

 have also been grown and used in crosses with one another and with 

 B. heegeri. In most cases these crosses seem to have been successful. 



BRISTLE INHERITANCE IN THE VINEGAR FLY, DROSOPHILA. 



Among the burning questions in genetics is the significance of char- 

 acters that seem not to be discrete units but to show all grades. Such a 

 character is skin color in man (Carnegie Inst. Wash. Pub. No. 195) and 

 size of body in rabbits and poultry. Recent studies indicate that the 

 apparent blends are due to the numerous factors involved in the 

 character. Another contribution to this subject has been made by 

 Dr. MacDowell, to whom the analysis of "Size Inheritance in Rabbits" 

 is due f Carnegie Inst. V\^ash. Pub. No. 196). In Drosophila there are 

 normally four bristles (2 rows of 2 bristles each) on the back of the 

 thorax. In other cases there may be one pair, or 2, 3, or 4 pairs of extra 

 bristles, or the extra bristles may be uneven in number. Dr. Mac- 

 Dowell's studies show that there is a Mendelian factor involved in the 

 inheritance of extra bristles, and as normal dominates extra it is 

 concluded that there is a dominant factor which restricts the number 



