66 



CHAPTER 9 



G, 



FIGURE 9 3. Results of matings 

 between yellow mice. 



From the biological standpoint, lethality 

 is characterized not by the absence of an 

 individual or a class of offspring, but by its 

 inability to reproduce. So, for example, a 

 genotype which causes complete sterility is 

 genically lethal, even should its possessor live 

 forever. Lethals which actually kill the 

 organism may act very early or very late in 

 development, or at any stage in between. 

 Sometimes a lethal effect is produced not by 

 one gene or a pair, but by the combined effect 

 of several nonallelic genes. In this case, some 

 of the nonalleles are contributed by each 

 parent, and the offspring dies because the 

 nonalleles, viable when separate, are lethal 

 when present together. 



Different alleles, recessive or dominant, 

 have been shown to affect viability to differ- 

 ent degrees. These effects cover the entire 

 spectrum — ranging from those which are 

 lethal, to those which are greatly or slightly 

 detrimental, to those which are apparently 

 neutral or even beneficial (Figure 9^). When 

 there is differential viability for different non- 

 alleles or alleles, phenotypic ratios may be 

 significantly modified from those expected. 

 The importance of the precautions to be 

 taken, relative to the viability and fertility of 

 the individuals bred in experiments designed 

 to establish principles of transmission genet- 

 ics, has been discussed in Chapter 2, and is 

 by now obvious. 



SUPRA-VITAL 

 (beneficial) 



1.3 



1^ 

 1.2 



NORMAL 



VIABILITY 



I 



1^ 

 1.1 



SUB-VITAL 

 (detrimental) 



J^ 



I r 



1.0 .9 .8 .7 .6 .5 

 RELATIVE VIABILITY 



t 



I 

 SUB-LETHAL 

 (semi-lethal) 



I 



LETHAL 



I I 

 I ' 



^A| 



FIGURE 9-4. Classification of 

 effects that mutants have on 

 viability. 



