MENDELIAN INHERITANCE IN OENOTHERA PRATINCOLA 7 



chance to express itself in the zygote because of the ever-present a factor 

 for flatness inherited from the pistillate parent. The only way for a revo- 

 lute-leaved plant to occur in the F 2 or following generations of the cross is 

 by an independent mutation from a to a in the presence of the recessive 

 condition of the Mendelian factors for flatness. Only one-fourth of the 

 plants of the F 2 generation, those with the constitution apff, are capable 

 of becoming revolute-leaved by mutation. Apparently there is nothing 

 to hinder mutation from a to a in strain C. But in pure strain C the 

 change would not be indicated by outward sign, for the strain is homo- 

 zygous for the Mendelian factors for flatness. That this change does 

 sometimes occur is shown by the few revolute-leaved plants which occur 

 in the F 2 generation of the cross f . typica C X mut. formosa (see table 8, 

 and COBB and BARTLETT 1919, table 6). In table 8 there are 26 revolute- 

 leaved plants in a total of 1654, or 16 per 1000. If the mutation to a' 

 should take place in a plant of the FI generation (a&Ff), a 3: 1 ratio would 

 occur in the F 2 generation. In the hope that this may sometime happen 

 in the experiment garden, the cross f. typica C X mut. formosa has been 

 repeated many times, and FI and F 2 progenies are being grown. 



THE FI AND F 2 GENERATIONS OF THE CROSS MUT. FORMOSA STRAIN E X F. 



TYPICA STRAIN E 



In the cross mut. formosa X f . typica E (afiff X aQff) the mechanism 

 for Mendelian inheritance operates just as certainly as in the correspond- 

 ing cross with f. typica C, but the two parents happen to be alike in the 

 Mendelian factors under consideration, both being pure recessives, so the 

 only type of inheritance which manifests itself is matrocliny, depending on 

 the difference in factorial composition of the characteristic portions of the 

 a and )8 gametes. This cross has been repeated successfully nine times, 

 giving 305 plants in the FI generation (see table 9) and 628 plants, from 

 four plants of the FI generation, in the F 2 generation (see table 10). All 

 of the plants of both generations were revolute-leaved. The inheritance 

 here is matroclinic, in contrast with the Mendelian inheritance in the cor- 

 responding cross with strain C. 



The reciprocal cross, f. typica E X mut. formosa (afiff X otfiff), has not 

 been successfully repeated since the publication (COBB and BARTLETT 

 1919) of the fact that this cross is also matroclinic, the number of revolute- 

 leaved plants occurring in the progeny being no greater than might be 

 expected from self-pollination of f. typica plants of strain E, the strain 

 which regularly produces some revolute-leaved plants in every generation. 



GENETICS 6 : Ja 1921 



