Multiple Alleles; Mucigenic Traits 



61 



and half of it cannot grow on its host. Re- 

 call that the stigma and style are diploid 

 tissues, whereas pollen grains are haploid. 

 These results suggest that most important in 

 determining whether or not a pollen grain 

 can grow down a style is not the diploid 

 genotype of its parent but the haploid geno- 

 type contained in itself. 



Let us assume that self- or cross-sterility 

 is due to a single pair of genes. Call s3 the 

 allele contained in the pollen which permits 

 pollen to grow in case B. The pollen grains 

 from the host plant furnishing the pistil can- 

 not contain s3, or the pollen would be able 

 to grow on their own parent; and they can- 

 not (Case A). So, the host pistil tissue in 

 this experiment cannot contain s3, and one 

 of its alleles can be called si. Then, half 

 of the pollen from the host individual will 

 carry si (Case A); but since these fail to 

 grow, we must assume that any pollen grain 

 carrying an s allele also present in the host 

 pistil will fail to grow. Excluding the pos- 

 sibility of a mutation, the other allele in the 



host pistil cannot also be 57, since one si 

 would have had to be received from a pa- 

 ternal pollen grain growing down a maternal 

 style that carried si as one of its two alleles. 

 Since the second allele in the pistils illus- 

 trated cannot be either si or s3, let us call 

 it s2. The other half of pollen from the 

 pistil parent thus will contain s2, and also 

 fail to grow in self-pollination (Case A). 

 In B the pollen grains that fail to grow are 

 either s/ or s2 (adhering to the law of par- 

 simony) ; however, their precise identity can- 

 not be determined without additional tests. 

 In C, since all the pollen grew, one pollen 

 allele must be a different one — call it s4. 

 The other pollen allele may be s3 or a still 

 different one, s5. Here again more tests are 

 needed to determine the precise identity. 



In these cases the phenotypic alternatives 

 for pollen are to grow or not to grow. 

 Whenever the pollen grains from any one 

 plant are placed on a given stigma and both 

 alternatives occur, the phenotypes are in a 

 1 : 1 ratio. All these results and others are 



Stigma 



VA *'% 



PISTIL < Style 



figure 5-4. Multiple alleles 

 for cross- or self -sterility. 



Ovary 



S1S2 



STS2 



S1S7 



