72 University of California Publications in Agricutlural Sciences [Vol. 2 



seedlings, which is a ratio of 2.67 to 1.33, a deviation from the 

 theoretical monohybrid ratio which might be expected to occur 

 under the laws of chance in 42 per cent of monohybrid popu- 

 lations. While positive statements will not be made until the 

 F x trees have been tested on a more extensive scale, yet a valid 

 3 to 1 ratio even with so few F 2 individuals is certainly an 

 indication that the genetic relationship between calif or nica and 

 quercina is a difference in a single factor of the -same Mendelian 

 reaction system. 



The idea that a single factor-difference may so affect the 

 entire chromosome system that the individual is altered more 

 or less in each somatic detail is now generally recognized, yet 

 the direct evidence on which this conception is based is found in 

 a limited number of cases. Morgan 2 refers to the mutant strains 

 of Drosophila ampelophila called "club" and "rudimentary", 

 in which the factor for a certain wing character also conditions 

 the development of certain other morphological and physiological 

 characters. But, thus far in Drosophila, no single factor has 

 been found that visibly affects every external feature of the 

 organism. The well-known dwarf or cupid sweet pea is a strik- 

 ing example of the manifold effects of a single factor. This 

 variety differs from the ordinary climbing form of Lathyrus 

 odoratus not only in its extremely dwarf stature but also in 

 color of foliage, length of internodes, size and arrangement 

 of flowers, time of anthesis, fertility and viability. Yet it is 

 certain that the variety differs from the species in a single 

 genetic factor. Probably this is as striking a case as has 

 been reported previously, yet in such a conspicuous character 

 as leaf -shape the dwarf variety very closely resembles the species 

 type. Now the oak-like walnut differs from the species type in 

 every gross external feature shape of leaves, color of foliage, 

 color of bark, habit of growth, structure of inflorescence, struc- 

 ture of flowers, size and structure of fruits, as well as in fertility 

 and viability. Therefore, if tests that are now being made 

 confirm our inference that quercina differs from the species in 

 a single genetic factor, it will be a most striking example of 

 the manifold effects of a single genetic factor. Further- 



2 Morgan, T. H., Mechanism of Mendelian Heredity, p. 209ff., 1915. 



