10. Effect of varying dosages of Dt. Previous data have shown that a non-linear effect was obtained when different dosages of Dt were present in the aleurone. However the demonstration of several modifying factors affecting the a-Dt reaction made it necessary to secure data bearing on this relationship in an iso-genic stock. Such an iso-genic stock was obtained through repeated self-fertilization of a Dt dt stock -- heterozygous Dt dt seed being used in every generation to further the inbreeding. After 5 years of selfing the F_{6} seed was classified into Dt Dt, Dt dt and dt dt classes. For the dosage relation between 1 and 2 Dt genes exact reciprocals were made between Dt Dt and dt dt plants. It was necessary to self Dt Dt individuals to obtain data on the effect of 3 Dt genes. The following data were obtained:

Mean number of mutations in | |||

Pedigree | Dt dt dt class | Dt Dt dt class | Dt Dt Dt class |

(1 Dt) | (2 Dt) | (3 Dt) | |

6134-13 × 6131.7 | 6.8 | 19.5 | |

reciprocally | |||

6134-6 × 6131-14 | 5.9 | 19.6 | |

reciprocally | |||

6134-1 × 6131-2 | 7.8 | 19.9 | |

reciprocally | |||

6134-2 × 6131-9 | 9.1 | 23.9 | |

reciprocally | |||

6385-24 × 6386-13 | 6.7 | 24.9 | |

reciprocally | |||

6385-9 × 6386-19 | 8.7 | 26.6 | |

reciprocally | |||

6389-11 × 6390-17 | 8.4 | 24.1 | |

reciprocally | |||

Mean ratio for 1 Dt : 2 Dt = 1 : 3 | |||

6131-18 selfed | 110.1 | ||

6131-8 selfed | 126.7 | ||

6396-2 selfed | 129.7 |

In each determination at least 50 seeds were used. The figures represent the average number of mutations (i.e. dots of color) in the aleurone layer. The mutation frequency in the 3 Dt class is too low. With such large numbers of dots per seed there is considerable overlapping of the mutant areas. Error also enters from the fact that an earlier mutation of one a allele will obscure a latter mutation of a second allele. In the case of 1 and 2 dosages of Dt this is an insignificant matter but it must be taken into account in considering the data from 3 doses of Dt. Due to the extreme difficulty in counting the dots on the 3 Dt class only 3 ears were counted. They were in no way different from the numerous uncounted ears of the same constitution. These data confirm the earlier conclusion that the effect of varying doses of the Dt allele is a non-linear one.