EFFECTS OF INBEEEDING AND CKOSSBREEDHSTG. 



41 



We have seen that the inbred lines of guinea pigs have actually 

 deteriorated in regard to all characters which have been studied. 

 Unfortunately we can not make a satisfactory comparison of the rate 

 at which this decline has taken place with theory because of the great 

 fluctuations from year to year which are evidently due to environ- 

 mental causes. We can, however, compare the records of the various 

 crossbreeding experiments with their theoretical relations to the 

 inbred average, since we have calculated all of them on the basis of 

 the inbreds raised simultaneously. 



Let X and y represent as before the relative proportions of any pair 

 of factors, A and a, in the original random-bred population. We will 

 start by assuming that the same proportions apply to the group of 

 inbred families which are used for crossing. This implies that there 

 has been no selection and also that enough families are taken to 

 represent adequately the original stock. The composition of the 



\ 



r 



JO 



Fig. 25. — The decrease in heterozygosis and correspondingly in vigor in successive generations of inbreed- 

 ing brother with sister, beginning with a random-bred stock (B, Al, A2, A3, etc.), or beginning with a 

 first cross between homozygous lines (CO, Cl, C2, C3, etc.). 



• 



first crossbred generation (CO) will then be x^AA^^xyAa^y^aa. 

 In spite of the apparent identity with the composition of the random- 

 bred stock, inbreeding of CO does not give the same result. This is 

 because there is necessarily a perfect correlation between brothers 

 and sisters in the first cross between lines assumed to have reached 

 homozygosis, while there is a correlation of only + .50 in the random- 

 bred stock. The results of three generations of brother-sister mating 

 from the first cross (Cl, C2, C3) are worked out in detail in Table 12 and 

 are compared with the effects of such mating in a random-bred stock 

 (or CC, CA, or AC) in Figure 25. There is a more rapid initial decline 

 in vigor on starting inbreeding from the first crossbred generation. 

 The first inbred generation (Cl) is composed in part of true breeding 

 lines {AA and ad) and in part of a group which is composed of \AA + 

 \Aa^\aa, like a random-bred stock in which x = y, but with zero 

 correlation between brothers and sisters. In the next generation 



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B 



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