Duplicate genes for capsule-form in Bursa bursa-pastorir- . 145 



Summary. 



The triangular form of capsule, characteristic of Bursa bursa-pastoris, 

 is produced by either of two independently inheritable Mendelian de- 

 terminers, C and D. When both of these are absent B. Heegeri appears, 

 having a small top-shaped capsule. The following facts have been dem- 

 onstrated in support of this conclusion: 



1. All individuals of the Fi families formed by crossing certain 

 American biotypes of B. bursa-pastoris with B. Heegeri, have triangular 

 capsules. 



2. In all F 2 families there is an approximation to the ratio 15 : 1 

 between plants having triangular capsules and those having top-shaped 

 capsules. 



3. When F 2 plants having triangular capsules are srelf-fertilized, 

 three kinds of families are produced: namely, a) those in which all of 

 the individuals have triangular capsules ; b) those in which the individuals 

 having the two kinds of capsules, bursa-pastoris and Heegeri, occur in 

 the ratio 15 : 1; and c) those in which the two kinds of plants appear 

 in the ratio 3:1. 



4. In the Fi the results differ according to the type of Fs family 

 to which the parents belonged: a) The members of those Fs families 

 which contained only plants with triangular capsules, when self-fertilized, 

 produce only triangular capsules again, i. e., they breed true to the 

 B. bursa-pastoris character; b) when the parents are triangular-capsuled 

 plants from an Fa family in which a 15 : 1 ratio occurred, the F 4 families 

 fall into the same three groups as the Fs families described above 

 under 3.; c) from the dominant individuals of an Fs family in which 

 a 3 : 1 ratio occurred only two kinds of F families arise, namely, 

 (i) with triangular capsules only, (ii) with the two kinds of capsules in 

 the ratio 3:1. 



5. Extracted dominants in the F 2 and later generations have not 

 all the same genotype as the original bursa-pastoris individuals used 

 in the crosses, though indistinguishable from them by inspection. A cross 

 between one of these extracted dominants and Heegeri has yielded 

 several F 2 families all of which gave a ratio 3 : 1 instead of 15:1 as 

 found in the original F 2 families. 



The deviations from the expected ratios were not as great as those 

 in the families reported in previous papers, and about as many families 

 have shown an excess of Heegeri as of bursa-pastoris, probably due to 

 the development of a more successful technique in handling the cultures. 



