The Journal of Heredity 



these crosses the 

 Fi generation is 

 as resistant as the 

 W h i t e Russian 

 parent and in the 

 Fo generation seg- 

 regation is ac- 

 cording to the 

 simple ratio of 3 

 resistant to 1 sus- 

 ceptible. A family 

 of F3 seedlings 

 was grown from 

 each of the 600 Fo 

 plants mentioned 

 above. Twenty- 

 five kernels of 

 each plant were 

 reserved for 1922 

 field planting and 

 25 or 30 of the 

 remaining kernels 

 were planted in 

 the greenhouse. 

 The kernels of 

 each individual 



plant were 

 planted separately in a 4-inch pot. The 

 seedlings were inoculated with stem 

 rust, and notes were taken about two 

 weeks after inoculation on the type of 

 infection obtained. Pots of White 

 Russian and Victory seedlings were 

 tested with each series of hybrid mate- 

 rial and the type of infection carefully 

 determined. 



The infection obtained on Victory 

 in the greenhouse seedling inoculations 

 is a 4±type4 (see Fig. 19). This is 

 the highest degree of infection obtained 

 on any variety. The infection ob- 

 tained on White Russian in the green- 

 house is a 3 ± type. Under the most 

 favorable epidemic conditions produced 

 artificially in the field, numerous small 

 uredinia, never larger than a pinhead, 

 may develop on White Russian. Un- 

 der ordinary field conditions in years of 

 severe rust epidemics. White Russian 

 is comparatively free from rust. A 

 very few small uredinia may develop 

 but no damage from them is apparent, 

 while susceptible varieties growing 



SEGREGATION FOR RUST RESISTANCE 



Figure 20. In the second hybrid gener- 

 ation segregation occurs — all the plants are not 

 alike, three that are resistant being found for 

 every rusted one. The self-fertilized, rusted 

 plants breed true, but only one third of the 

 resistant plants do so. The progeny of the 

 other two thirds segregating again in the next 

 generation. It is impossible to tell the pure 

 resistant (homozygous) plants from the impure 

 (heterozygous) ones, so to separate them an- 

 other generation must be grown. (See text, p. 

 187.) 



in adjacent rows 

 become very 

 heavily infected, 

 the culms being 

 literally covered 

 with confluent 

 uredinia. 



The number of 

 F3 families which 

 bred true for re- 

 sistance came 

 very close to ex- 

 pectation (see 

 Table I). In the 

 cross White Rus- 

 sian X Victory, 82 

 F3 families out of 

 229 families 

 tested bred true 

 for rust resis- 

 tance. This gives 

 a ratio, per 3 of 

 1.07:1.93 which 

 is very close to 

 the expected 1 :2 

 ratio. In the cross 



Minota x White 

 Russian and the reciprocal, 110 families 

 out of 338 tested bred true for rust 

 resistance. This is a ratio, per 3, of 

 1.01:1.99. 



Since it is possible to obtain large 

 numbers of homozygous resistant 

 plants, the heterozygous F2 plants 

 may be eliminated on the basis of F3 

 seedling tests in the greenhouse. 



All families in which segregation 

 occurred in F3 were grouped and the 

 ratio of resistant to susceptible seed- 

 lings was calculated (see Table II). 

 The deviation from the expected 3:1 

 ratio is quite large in each case. The 

 discrepancies are no doubt due to errors 

 in taking data. The type of infection 

 obtained on White Russian is a 3 + 

 type, and that obtained on Victory 

 or Minota is a 4+ type (see Fig. 19). 

 It is quite possible that in some cases 

 the uredinia had not fully developed 

 when notes were taken and accordingly 

 some seedlings were classed as a 3 + 

 type which a day or so later would 

 have been a 4+ type. 



* The use of these symbols was suggested by Stakman and Levine. 



