334 BIOLOGY OF THE LABORATORY MOUSE 



Thus, average vigor and average fertility usually decline with inbreeding 

 (13). The generally accepted explanation of this phenomenon is that genes 

 unfavorable to vigor, fertility, etc., are usually recessive; and, since inbreed- 

 ing fixes genes in homozygous condition, irrespective of whether they are 

 dominant or recessive, all the individuals in an inbred line are likely to have 

 a proportion of homozygous recessives higher than that in the average 

 individual in the foundation stock. 



Effect of Inbreeding on the Variation 



Causes of a change in variation. — It was stated in the introduction to 

 this chapter that inbreeding may lead either to a decrease or to an increase 

 in phenotypic variation. In order to explain both effects it is necessary to 

 distinguish as separate causes: (7) the reduction of genetic variation, and (2) 

 the characteristics of the resulting fixed genotype. Let us consider each of 

 these in turn. 



1. It is apparent from Fig. 134 that reduction in genetic variation will, in 

 itself, tend to decrease variation in a character. In the limiting case, when 

 all genes affecting a character have become fixed, differences between 

 individuals will be determined solely by differences in the ''physical 

 environment." 



2. The characteristics of the genotype fixed may, however, tend either 

 to decrease, or to increase, phenotypic variation, according as the develop- 

 mental processes determined by this genotype are less, or more, susceptible 

 to variation in the "physical environment" than are the developmental 

 processes of the bulk of the individuals in the foundation stock. 



The total effect of inbreeding on phenotypic variation will be due to a 

 combination of (7) and (2). If the tendency of (2) is either to decrease 

 variation in a character, or not to increase it as much as it is decreased by 

 (7), then the character will be less variable in the inbred line. On the other 

 hand, if (2) increases the variation more than (7) tends to reduce it, then the 

 inbred line will show more variation than the foundation stock. 



Here, then, we have an explanation of the paradox, mentioned in the 

 introduction to this chapter, that, although inbreeding causes a reduction in 

 genetic variation, it sometimes results in increased variation in a char- 

 acter. The result is due to the nature of the genotype fixed.* 



* From a statistical point of view the paradox can be explained by the type of scale 

 on which the variation is measured. The increase in variation can occur only when 

 the scale is such that the magnitude of the environmental effects differs at different 

 points on the scale. An opportunity is then provided for inbreeding to shift the stock 



