70 



CHAPTER 6 



FIGURE 6-2. Classification of c fleets that mu- 

 tants have on viability. 



merit, or at any stage in between. Some- 

 times a lethal effect is produced not by one 

 gene or a pair, but by the combined effect 

 of several nonallelic genes. In such a case, 

 some of the nonalleles are contributed by 

 each parent, and the offspring dies because 

 the nonalleles, viable when separate, are 

 lethal when together. 



Different alleles, recessive or dominant, 

 have been shown to affect viability in dif- 

 ferent degrees. These effects cover the en- 

 tire spectrum — ranging from those which are 

 lethal, through those which are greatly or 

 slightly detrimental, to those which are ap- 

 parently neutral or even beneficial (Figure 

 6-2). When different combinations of 

 alleles or nonalleles have different viabilities, 

 the phenotypic ratios observed may differ 

 significantly from those expected. The im- 

 portance of the precautions to be taken, 

 relative to the viability and fertility of the 

 individuals bred in experiments designed to 

 establish principles of transmission genetics, 

 has already been discussed in Chapter 3. 



Pleiotropism 



Does each gene affect only one trait or can 

 it have multiple, manifold, or pleiotropic 

 effects? 



An investigation ' can be undertaken to 

 answer this question, using two strains of 

 Drosophila that are practically identical ge- 

 netically (isogenic), except that one is pure 



1 See Th. Dobzhansky and A. M. Holtz ( 1943 ). 



tor the gene for dull-red eye color (vv+) 

 and the other is pure for its allele white (w). 

 Another trait is then chosen for examination 

 in these two strains, one apparently uncon- 

 nected with eye color. The trait selected is 

 the shape of the spermatheca, an organ found 

 in females which is used to store the sperm 

 received. The ratio of the diameter to the 

 height of this organ is determined for each 

 of the two strains. This index of shape is 

 found to be significantly different in the dull- 

 red as compared to the white strain. From 

 this result it can be concluded that the eye- 

 color gene studied is pleiotropic. The re- 

 sults of other studies also show many differ- 

 ent genes to be pleiotropic for morpholog- 

 ical traits. 



In Drosophila a recessive lethal gene 

 called lethal-translucida causes pupae to be- 



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figure 6-3. Pleiotropism at the biochemical 

 level. (After E. H adorn.) 



