SYSTEMS OF MATING 



39 



The following assertions are intended to help mammalian geneticists in the task 

 of choosing a suitable breeding system. 



1. Close inbreeding, such as brother-sister, has been successful on a large scale 

 only with the house mouse. There are in existence, however, lines of rats, rabbits, 

 guinea pigs, and hamsters which satisfy the usual working definition of being inbred, 

 that is, they have survived 20 or more generations of exclusive brother-sister inbreeding. 

 The house mouse does not, however, withstand the depressing effects which accompany 

 inbreeding as well as the existing profusion of inbred strains suggests. The existing 

 strains are the successful survivors of, probably, a twofold or threefold larger number of 

 attempts to establish inbred lines. If one were to start to produce a new inbred strain 

 of mice, he should maintain several lines, say five, to insure that one or two can be 

 propagated to 20 generations at least. 



2. Any of the systems of mating (except random mating) described in this chapter 

 will increase the probability of incrosses and will decrease the probability of heterozygo- 

 sity at all loci except those closely linked with a locus of interest deliberately forced to 

 remain heterozygous. Table 1 1 shows the probabilities of incrosses after 12 generations 

 for each of the regular systems. For loose linkage (c between 3/10 and 1 12), the methods 

 of crossing with locus-control only are all more efficient than the methods of inbreeding 

 and locus-control, assuming that the inbred strains used in crossing are in fact homo- 

 zygous. For closer linkage, the methods using forced heterozygosis have greater 

 efficiency. Efficiency may be defined by the number of generations required to achieve 

 a given probability of incrosses or by the probability of incrosses achieved with a fixed 

 number of generations. Table 1 1 also shows the number of generations required to 

 obtain a 95 per cent or a 99 per cent frequency of incross matings. 



Table 1 1 



Probabilities of incrosses after 12 generations and numbers of generations 

 required to achieve probabilities of 95 and 99 per cent for incrosses for six 



mating systems 



System 



Probabilities of incrosses at G 12 



Generations required to 



obtain a per cent of 



incrosses 



= 0.5 



0.4 



0.3 



0.2 0.1 



ac = 0.5 0.4 0.3 0.2 0.1 



2. Brother x sister 



3. Backcross 



4. Cross-intercross 



0.8922 — 



0.9995 0.9964 0.9802 0.9141 0.6862 



0.9740 0.9267 0.8282 0.6508 0.3727 



0.9932 0.9725 0.9130 0.7718 0.4877 



5. Cross-backcross- 



intercross 



6. Heterozygosis forced 0.8922 0.8853 0.8618 0.8097 0.6833 



by backcrossing 



7. Heterozygosis forced 0.8922 0.8920 0.8890 0.8723 0.7953 



by intercrossing 



