274 Multiple Alleles 



ing Nicotiana, homozygotes can be obtained by self-pollinating 

 young buds instead of mature pistils. 



The results of crossing two self-sterile but interfertile plants 

 are very interesting. If the two parents have no allele in com- 

 mon, four intrasterile, interfertile classes will be produced. If a 

 plant of the constitution s^s^ is pollinated by an s^s* plan, the 

 four classes will be sV, s^s^, sV, and sV. Thus, if 100 off- 

 spring were raised, we should expect theoretically 25 plants of 

 each class. All the plants would be self-sterile but each plant 

 would be fertile with only 75 out of the other 99 plants, for it 

 would be cross-sterile with the other 24 plants of its own class. 



If the two plants which are crossed have one self-sterility al- 

 lele in common, only two classes are produced in the offspring, 

 and they should appear with equal frequency. The two classes 

 from such a cross, however, are not both the same as the two 

 that are produced by the reciprocal cross. The following two 

 crosses illustrate this point: 



sh- X sh^ = s^s^ + s^s^ 



s^s^ X s^s^ = s^s^ + s^s^ 



It will be observed from this that if a common allele is present, 

 the class of the mother is never represented in the offspring. 



If an s^s^ plant is self-pollinated in the young bud, fertilization 

 may result. Three classes will be produced in the offspring, and 

 one of them will contain half the individuals. Since neither the 

 s^ nor the s^ pollen may grow sufficiently slowly to prevent fer- 

 tilization after a bud pollination, s^s"^ X s^s^ in the young bud 

 will give Is^s^: 2s^s^ : s^s-. If plants of these three classes are 

 backcrossed to the parent, those of the second type will be 

 sterile with the parent in both directions. When the parent is 

 the female, each of the other two classes wdll fail to set seed with 

 it if pollinated normally. If, however, the parent is the male, 

 fertility will be the result with .plants of each of these homozy- 

 gous classes. The establishment of homozygous types is very 

 useful in identifying classes. Figure 80 shows the results of 

 crossing plants of various normal and homozygous classes. 



If genes for various morphological characters are linked with 

 the genes for self-sterility, the ratios will be greatly disturbed 



