18 SIZE INHERITANCE IN RABBITS. 



in F 2 gave some plants with spikelets all on one side (Fahnentypus). 

 Kajanus (1911) reported three crosses between different long types of 

 Beta, which gave similar forms in the first generation, while in the 

 second generation very long individuals were obtained. Balls (1907), 

 in crossing different strains of cotton with plants of the same height 

 (60 cm.), found a great variability in the heights of F 2 . Fi was uni- 

 formly close to 100 cm.; F 2 ranged from 30 cm. to 100 cm., "although 

 the plants had been practically level until two months old" (p. 365). 

 Hayes (1912) has crossed varieties of tobacco with similar numbers of 

 leaves which gave F,. plants with the same number of leaves and F 2 

 plants with great variability in the number of leaves. 



TRANSGRESSING SEGREGATION. 



In certain cases quantitative morphological differences give imperfect 

 monohybrid ratios; three classes corresponding to 1 :2:1 may be found 

 with every intergrade between them. In crosses involving different 

 glume lengths in wheat Biff en (1905) found overlapping curves of this 

 sort. Plants in F 2 with long and short glumes gave long and short 

 glumes, respectively, in F 3 , while intermediates gave all classes. Biffen 

 also describes continuous multimodal curves of the F 2 frequencies from 

 crosses of barley with heads of different lengths. Balls (1907) found a 

 uniform Fi, taller than the tall parent, in a cross between cotton plants 

 of different heights. The range of the second generation exceeded that 

 of either parent and the frequency curve exhibited three modes, sug- 

 gesting the monohybrid ratio. In crosses involving weight of seed he 

 obtained bimodal curves in F 2 . The seed weights in the parental lines 

 were 0.10 grams and 0.125 grams; F x = 0.155 grams; F 2 = 0.08 — 0.16 

 grams. " Small seed segregates from large seed, and the segregated types 

 show a greater range of fluctuation than is shown by either parent" 

 (p. 372). 



Another case of incomplete segregation into three classes is given by 

 Iveake (1911) in cotton leaves. The ratio of the width to the length 

 of the central lobe of the leaf was called the leaf factor. For a race 

 this leaf factor was found to be extremely constant. Crosses between 

 races with different leaf factors gave intermediate offspring which were 

 as uniform as either parent. (See a plate given by Allard, 1910). 

 These inbred gave an increased variability in the leaf factor, forming 

 a continuous series all the way between the extremes of the parents. 

 The greatest frequency was half way between the original parents and 

 at the mode of the first generation frequencies. But there were two 

 lesser modes in the curve that corresponded to the modes of the parental 

 frequencies. The ratio of individuals in these three groups was 1:2.2:1. 

 The third generation showed that most of the individuals about the 

 secondary modes remained within the parental ranges, while the plants 

 about the large mode repeated the multimodal curve shown by the 



