38 GENETIC STOCKS AND BREEDING METHODS 



Fig. 12. Probability of heterozygosity with 



HETEROZYGOSIS FORCED BY INTERCROSSING. 



PROBABILITY 

 h„ 



2 4 6 8 

 GENERATION n 



The probability of heterozygosity h n for selected values of c = 1/10, 1/2 for the system 

 of brother-sister inbreeding with heterozygosis forced by intercrossing, starting with q = 1 . 



where // = 2X and / = 2c(l — c). Solutions for selected values oft give the following 

 characteristic roots: 



c = 0.5 0.4 0.3 0.2 0.1 



Xj = 0.8090 0.8092 0.8115 0.8235 0.8674 1. 



Again the equation 



|G - XI | = (8) 



has not been solved, so that formula 



V n = A»V (6) 



cannot be applied. However, the above value of X x can be used to estimate the num- 

 ber of generations required to exceed 95 per cent and 99 per cent incross matings. 

 These are, for c = 1/2, 15 and 22 generations, and for c = 1/10, 22 and 33 generations. 

 Again it should be noted that all three genotypes at the locus of interest are produced 

 on the same genetic background. 



Examples of strains of mice produced by this system are: WB/Re-W T , 

 WC/Re-W, WH/Re-H 7 , and WK/Re-M'. These four strains in which all matings 

 are Ww x Ww provide four different genetic backgrounds on which to compare 

 three genotypes, WW, Ww, and ww of the dominant spotting locus. 



GENERAL REMARKS 



The breeder of laboratory animals for research must face the question of how to 

 produce and propagate animals of the specific types he needs for his specific objective. 

 The choice will depend upon the type of animal, the knowledge of the genetics of the 

 trait of interest, the relative efficiency of the mating systems, the ease or difficulty of 

 determining the phenotype of each animal, and the amount of space in the animal room 

 to be devoted to maintenance of stocks. 



