18 Mutations and Evolution. 



(E. nanella x lamarckiana and its reciprocal, where both parent 

 types split out in the F t generation and both subsequently breed 

 true, it is reasonable to conclude that some determining reaction 

 occurs in the fertilized egg, in which one parental germ cell or the 

 other gains the ascendancy. This was formerly explained by 

 De Vries on the basis of pangens in different conditions. But 

 since the important work of Muller (1918) on balanced lethal factors 

 in Drosophila, it is evident that linkage to such lethal factors may 

 explain this result, though more will need to be known concerning 

 such lethal factors in (Enothera before the explanation can be 

 applied in detail. Differential sterility depending on whether the 

 lethal factor is linked to (i.e., is in the same chromosome with) the 

 factor for dwarfness or that for tallness, would seem to meet the 

 case. 



Concerning the origin of (E. lamarckiana mut. nanella, we may 

 assume that it appears through the breaking of the linkage between 

 the dwarfing factor and a lethal factor, for if it were a simple 

 Mendelian recessive we should find occasionally a Lamarckiana 

 plant which was heterozygous for dwarfness and gave 25% dwarf 

 offspring. But such are never found in lamarckiana although, as 

 we have already pointed out, they do occur in (E. gigas. This 

 suggests that gigas has lost some of the lethal factors present in 

 Lamarckiana. As a mutant, nanella has a frequency of only 1 2%, 

 so that the cross-over between the dwarf and lethal factors must be 

 an infrequent one. The discussion of lethal factors will be taken 

 up again later. 



That mutations in other forms may date their origin from 

 fertilization has also been held, and Punnett (1919) has recently 

 concluded that this is the time of origin of the well-known cretin 

 mutation in the sweet pea. This mutation differs prominently 

 from the type in having a straight stigma protruding through a cleft 

 keel, and it is invariably sterile on the female side. It behaves in 

 crosses with the parent as a simple Mendelian recessive, a total of 

 80 families in six years giving 4198 normals : 1322 cretins, which is 

 not far removed from a 3 : 1 ratio, the viability of the cretin being 

 evidently somewhat less than that of the normal form. 



The cretin appeared in a cross between two white sweet peas, 

 Blanche Burpee with long pollen and Emily Henderson with round 

 pollen. Prom three purple F t plants large F 2 families were raised 

 in 1905. Prom one of these P 2 families containing 187 plants, the 

 seed of 29 individuals was saved for the F 3 generation, These 29 



