No. 567] NOTES AND LITERATURE 



189 



A and B are single factor characters. In the majority of char- 

 acters, the F x hybrids would be intermediate or possess those of 

 either one or the other parent, since all the F x individuals would 

 be alike as far as any hereditary quality symbolized by XY is con- 

 cerned, providing the plants were all grown under the same en- 

 vironmental conditions. But these F t individuals would not be 

 alike as regards the inheritance of the characters A and B. Ex- 

 perimental evidence from crosses of this kind show us that four 

 different P, forms may result, the distinctions between them aris- 

 ing from the presence or absence, through inheritance, of the 

 characters A and B. Dominance is assumed to be absent in this 

 illustration. 



Swingle's Citrus hybrids, though involving greater complexity 

 because a large number of parental characters instead of two are 

 probably heterozygous, are of the same general type as those of 

 the illustration and lend themselves to the same interpretation. 

 Owing to the absence of sufficient exact experimental data, one 

 can not speak of unit characters and factors in these hybrids, but 

 one may say without violence to modern theories of heredity that 

 one or both of the parents involved in the crosses which produced 

 the Colman and the Cunningham were heterozygous in the factors 

 or factor for pubescence, that various size factors were hetero- 

 zygous and that one parent was homozygous for absence and one 

 for presence of the factors for hardiness, compound leaves and 

 evergreen foliage. 



F x variation in Citrus hybrids then, in the light of the data at 

 hand, apparently results from differences in the gametic compo- 

 sition of the heterozygous parents. 



Swingle calls attention to other cases of variation in F, hy- 

 brids from two pure stocks which support his contention that this 

 phenomenon of F x variation is very general, though usually 

 obscured through variation due to heterozygous parent stock. 

 Collins and Kempton 3 crossed a race of corn breeding true to 

 waxy endosperm with one constant for horny endosperm. Horny 

 endosperm was dominant in F t and the F 2 generation segregated 

 in the expected ratio of 1 waxy to 3 horny kernels. This ratio 

 represented the average proportion of each when the ears of all 

 the plants were lumped together. The F 2 progeny of each selfed 



8 Collins, G. N., and Kempton, J., II, 1912, "Inheritance of Waxy Endo- 

 sperm in Hybrids of Chinese Corn," IV° Conf. Internat. de Genetique, 1911, 

 P- 347; also Circ. No. 120, Bur. of P. I., U. S. Dept. of Agr., 1913. 



