274 GENETICS IN RELATION TO AGRICULTURE 



produced 39 variegated ears and 36 pure white ears, which is clearly 

 a 1:1 ratio in each case. This proves that both types of grains were 

 heterozygous for a dominant mutant factor and that both of the factor 

 mutations occurred in only one member of a duplex pair of factors. Pre- 

 sumably the mutation from white to variegated occurred first, and later 

 the mutation from variegated to red in a cell so located that, as the shoot 

 developed, only a portion of the ear was affected. 



There appears a very important obstacle to the conception of "somatic 

 segregation" in that the mechanism of cell division is apparently one of 

 the most nearly perfect and regular of natural systems and that the order- 

 liness of procedure is especially notable in undifferentiated tissue, where 

 bud sports and chimeras commonly originate. To assume that the oc- 

 currence of self-colored flowers on variegated plants is due to chromosome 

 aberrations in mitotic divisions is much less plausible than to explain such 

 phenomena by assuming a simple factor difference as responsible for self- 

 color and variegation, and that changes from one state to the other are 

 possible under certain conditions. This is the only reasonable hypothesis 

 by which to explain mutations from the recessive to the dominant condi- 

 tion of a pair of factors, as we have seen in the case of Hartley's ear of red 

 and variegated corn. Therefore, while chromosome aberrations are 

 known to occur during mitosis and aberrant numbers of chromosomes 

 have been found in senile and diseased tissues, yet, in general, bud sports 

 and chimeras are satisfactorily explained on the basis of factor mutations; 

 whereas "somatic segregation" as the term has been used by Bateson, 

 Gates and others implies the common occurrence of breaks in the 

 mechanism of mitosis such as are not known to occur in normally 

 functioning somatic cells. 



It should be remembered that horticultural literature contains nu- 

 merous peculiar cases of discontinuous variation, many of which have been 

 described or "explained" as "somatic segregations" resulting from 

 hybridization. We believe that most of these cases can be explained 

 much more reasonably in terms of factor mutations. But certain 

 discontinuous variations in plants are undoubtedly the result of neither 

 factor mutations nor chromosome aberrations in vegetative tissues. 

 For example, persistent and deciduous calyx lobes are sometimes found 

 on fruits of the same plant especially in the rose family. Tufts has 

 described the occurrence of this phenomenon in the Le Conte pear and the 

 Transce.ndant crab-apple as "somatic segregation," assuming that some 

 sort of segregation-mechanism exists in the division of somatic cells. 

 Data from the pear tree gave a ratio of 3.15 deciduous to 0.85 persistent 

 lobes. But to assume irregularities in chromosome behavior such as 

 would cause segregation preceding the formation of nearly one-fourth of 

 the calyx lobes on the tree is unwarranted in view of the general regularity 



