MUTATION. 169 



removed, or all three as the case may be. The very best ex- 

 ample of such an instance is the one which Shull discovered in 

 Capsella. Shull found that several normal Capsellas, with tri- 

 angular fruit, differed from a form with roundish fruit 

 (Heegeri) because of the possesion of three distinct genes more. 

 He further found, that each one of these three genes was in 

 itself sufficient to produce the difference, sufficient therefore 

 in this instance to "determine" the triangular shape of the 

 fruit. If we call these genes A, B and C, plants which have only 

 A grow up normal, and so do plants with only B or C, or any com- 

 bination of these three genes. In fact, only those plants which 

 lack all the three genes have the roundish fruit. The result is, 

 that if Shull crossed a plant with normal fruit to one with the 

 oval capsules, the hybrids had normal fruit, and the propor- 

 tion of "Heegeri" plants segregating out of the second genera- 

 tion varied according to the presence in the normal grand- 

 parent of one, two, or three of the genes A, B, and C. Some F 2 

 families would give one plant with oval fruit out of every four, 

 others produced one plant Heegeri out of every sixteen, and in 

 other families only one individual in every sixty-four would 

 reproduce the recessive character. Honing found an analogous 

 case in Canna. 



Now one of the results of this fact, that either of three differ- 

 ent genes is sufficient for the production of a certain domin- 

 ant character, is, that the recessive character will inevitably 

 be produced in F2 if two plants are bred together which are 

 both normal, but do not happen to have either of the three 

 genes, A , B, or C in common, for instance if we cross a plant 

 with A to one having B and C only. And we will see how this 

 cause of the production of recessive novelties from parents, 

 pure for a corresponding dominant character can be very easily 

 mistaken for real spontaneous loss of a gene, real loss-muta- 

 tion. In fact, it is only prudent to say that in all animals and 

 obligatory allogamous plants only very elaborate test-matings 

 can make an alleged case of mutation stand as proved. 



In Shull's work with Capsella, the number of parallel genes 



