SYSTEMS OF MATING 41 



differences between some of the inbred strains. Yet, once found, there may be great 

 difficulty in accounting for the difference in genetic terms. If such an analysis is im- 

 portant, a classical analysis of producing first, second, and third hybrid generations and 

 first and second backcross generations may yield the information that two given strains 

 differ by 8 or 10 or 50 pairs of genes. This information does not, however, easily pave 

 the way for developmental, physiologic, and other studies with precise genetic control. 



SUMMARY 



Mammalian geneticists use a number of breeding techniques to produce suitable 

 animals for research. Aside from random mating, the common objective of all of the 

 systems of matings described in this chapter is to reduce the probability of heterozygotes 

 and to increase the probability of matings of like homozygotes with respect to any locus, 

 called the a-locus, which is not being specifically controlled in the mating system. 



The random mating system is intended to hold constant the probabilities of all 

 genotypes at each locus from generation to generation. 



The brother-sister inbreeding system will increase the probability of incrosses, 

 that is, of matings of like homozygotes, with each advancing generation. For neutral 

 genes, the probability will exceed 95 per cent after 1 6 generations and 99 per cent after 

 24 generations. 



Three systems of mating (the backcross system, the cross-intercross system, and the 

 cross-backcross-intercross system) are all means of placing mutant genes and their non- 

 mutant alleles on inbred backgrounds. The backcross system is especially useful with 

 dominant mutations, but it may be used with recessive viable or recessive lethal 

 mutations. The cross-intercross and cross-backcross-intercross systems were designed 

 for putting recessive viable mutations on inbred backgrounds. The cross-backcross- 

 intercross system is more efficient than the cross-intercross system in that it requires 

 fewer generations to reach a specified probability of incrosses with respect to any neutral 

 locus (a-locus) not being specifically controlled. 



Two systems of mating combine inbreeding with locus control. These are brother- 

 sister inbreeding with heterozygosis forced by backcrossing and brother-sister in- 

 breeding with heterozygosis forced by intercrossing. The first, or backcrossing 

 system, is useful with a semidominant lethal or recessive viable mutation ; the second, or 

 intercrossing system is useful with recessive lethal mutations. When the neutral a-locus 

 is closely linked with the mutant locus (0 < c ^ 0.2), these two systems are more 

 efficient than the three systems which require locus control accompanied by crossing 

 with an inbred strain. For higher values of crossing over, they are less efficient. 



DISCUSSION 



Dr. Burdette: One most qualified to discuss this paper is Dr. Sewall Wright, 

 who has consented to review from a different point of view the problems of breeding. 



