EFFECTS OF INBREEDING AND CROSSBREEDING. Al 
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 z 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 
Np 
50 
PLEA CLIV7— 
Fic. 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 (C0, Cl, C2, C3, etc.). 
first crossbred generation (CO) will then be 2?4A+2ryAa+y7/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 (C1, 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 OC, CA, or AC) in Figure 25. There is a more rapid initial decline 
j2 vigor on starting inbreeding from the first crossbred generation. 
The first inbred generation (C1) is composed in part of true breeding 
lines (AA and aa) and in part of a group which is composed of }4.A + 
4Aa-+ jaa, like a random-bred stock in which z=y, but with zero 
correlation between brothers and sisters. In the next generation 
