64 



CHAPTER 9 



approximately all, respectively, were able to 

 grow down the style of the host. Note, in 

 B, that although all the pollen used came 

 from one diploid individual, half of it could 

 and half of it could not grow on its host. 

 It is known that the stigma and style are 

 diploid tissues, while pollen grains are hap- 

 loid. These results suggest that what is im- 

 portant in determining whether or not a pol- 

 len grain can grow down a style is not the 

 diploid genotype of its parent, but the haploid 

 genotype 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, 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 failed to grow, we 

 must assume that any pollen grain carrying 

 an s allele also present in the host pistil will 

 fail to grow. Ignoring mutation, the other 

 allele in the host pistil cannot be si, too, 

 since one si would have had to be received 

 from a paternal pollen grain growing down a 

 maternal style that carried si as one of its 

 two alleles. Since the second allele in the 

 pistils illustrated cannot be either ^7 or s3, 

 let us call it s2. So, the other half of pollen 

 from the pistil parent will contain s2, and 

 also fail to grow in self-pollination (case A). 

 In B the pollen grains which fail to grow are 

 either si or s2, employing the law of parsi- 

 mony, but which they are cannot be deter- 

 mined 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, 

 and a decision on this requires additional 

 tests. 



In these cases the phenotypic alternatives 



for pollen are to grow or not to grow. When- 

 ever the pollen grains from any one plant 

 are placed on a given stigma and both alter- 

 natives occur, the phenotypes form a 1 : 1 

 ratio. All these results and others are con- 

 sistent with the assumptions made, that self- 

 or cross-sterility is regulated by a single pair 

 of genes which form a multiple allelic series. 

 Some species have fifty or more multiple 

 alleles forming a series responsible for self- 

 sterility, or group sterility, or group incom- 

 patibihty. 



Wing Venation in DrosophUa 



In different wild populations of DrosophUa, 

 which we may designate as 1,2, and 3, the 

 venation on the wings is complete and identi- 

 cal, and when all possible hybrids are made 

 between these populations, the venation re- 

 mains unchanged. This result would suggest 

 that all three populations are genotypically 

 identical in this respect. On the other hand, 

 there is a mutant strain in which the venation 

 is incomplete, the cubitus vein being inter- 

 rupted {ci = cubitus interruptus) in homo- 

 zygotes (Figure 9-2). Hybrids, formed by 

 crosses between ci and wild populations 1 or 

 2, are found to have complete venation, so 

 that the gene for normal venation, c/+, in 



FIGURE 9-2. Normal (a) and cubitus interruptus 

 (b) >vm^5o/Drosophila melanogaster. {Courtesy 

 ofC. Stern; by permission of Genetics, Inc., vol. 

 28, p. 443, 1943.) 



