60 



CHAPTER 5 



figure 5-3. Normal (a) and cubitus inter- 

 rupts (/>) wings of Drosophila melanogaster. 

 (Courtesy of C. Stern; from Genetics, vol. 28, 

 p. 443, 1943.) 



an effect of this gene pair rather than a 

 modifying effect of some other gene pair. 

 Apparently, then, the ci + allele in popula- 

 tion 3 is different from that in populations 

 1 and 2. We are dealing, consequently. 

 with two isoalleles in a multiple allelic series. 

 (Note that a slightly different system of 

 symbolizing genes was used here.) 



4. Eye Color in Drosophila. Another series 

 of multiple alleles in Drosophila involves eye 

 color. In this case the different alleles can 

 be arranged in a series that shows different 

 grades of effect on eye color, ranging from 

 dull-red to white: dull-red (w + ), blood 

 (u'-'). coral (W"), apricot (u"'). buff (w 6/ ), 

 and white (w). The w * allele is dominant 

 to the others listed and is the allele com- 

 monly found in wild-type flies. One can 

 think of all the different alleles as producing 

 the same kind of phenotypic effect, but less 

 of it in proceeding from w + to w, the white 

 allele being completely inefficient in this 

 respect. 



We have already described isoalleles for 

 genes normally expressed in individuals liv- 

 ing in the wild (wild-type isoalleles). Iso- 

 alleles for mutant genes (mutant isoalleles) 

 also occur. For instance, it has been proved 



that the gene producing white eye color in 

 different strains of Drosophila is actually 

 composed of a series of multiple isoalleles 

 (vv 1 , w'-, w :! , etc.). 



5. Self-sterility in Nicotiana. Among sex- 

 ually reproducing plants it is not uncommon 

 to find that self-fertilization does not occur 

 even though the male and female gametes 

 are produced at the same time on a given 

 plant. The reason for this has been studied 

 in the tobacco plant, Nicotiana, where it was 

 found that if pollen grains fall on the stigma 

 of the same plant, they fail to grow down 

 the style. When this happens self-fertiliza- 

 tion is impossible. A clue to an explanation 

 for this phenomenon comes from the ob- 

 servation that different percentages of pollen 

 from a completely self-sterile plant may 

 grow down the style of other plants. 



The results of certain crosses are shown 

 in Figure 5-4. Genetically identical pistils 

 are exposed to pollen from the same plant 

 (A), from a second one (B), and from a 

 third (C). No pollen, approximately half, 

 and approximately all, respectively, are 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 can 



