18 



GENETIC STOCKS AND BREEDING METHODS 



THE BACKCROSS SYSTEM 



The backcross system is a means of placing a dominant mutation or a semidominant 

 lethal mutation D on a standard inbred background. It is assumed that the standard 

 inbred strain is A Add or aadd and the mutant bearing stock is — DD or — Dd. All 

 matings are backcrosses dd x Dd with respect to the ZMocus and may be incrosses, 

 crosses, or backcrosses, with respect to the a-locus. The backcross system may also be 

 used to put a recessive lethal or a recessive with low viability or low fertility on an inbred 

 background, by backcrossing heterozygous carriers Rr of the mutation r to a standard 

 inbred strain RR. In this case it is necessary to be able to distinguish the heterozygotes 

 Rr from the homozygotes RR, each suspected carrier R- being tested by matings with 

 known heterozygotes Rr. 



The backcross system is shown in figure 3. 



Fig. 3. The backcross system. 



INBRED 

 STRAIN 



MUTANT 



INBRED 

 STRAIN 



MUTANT MUTANT 



GENERATION 







3 co 



GENERATION 



CO 



I OO 



A. Backcrossing with a dominant or semidominant lethal mutation, D. 



B. Backcrossing with a recessive lethal mutation, r. 



