INBRED AND HYBRID ANIMALS AND THEIR VALUE 327 



The few technical terms and simple genetic concepts not explained can be 

 understood by reference to a text-book on the subject. 



Genetic Effects of Inbreeding 



Following Mendel's work, studies on the mechanism of heredity were 

 naturally focussed on mutations that produced easily recognizable effects. 

 This emphasis on major mutations invited the conclusion that nearly all 

 individuals in any one species have the same genotype (set of genes), the 

 remaining individuals exhibiting mutations. Such is not the case. Genetic 

 studies have shown that in, for example, any wild population of rodents, or 

 any laboratory population not closely selected or inbred, there is tremendous 

 genetic variation; although the population may show none of the major 

 mutations recorded by the geneticist. The changes effected by selection of 

 small phenotypic variations may be cited as one demonstration of this fact. 



Before examining the effect of inbreeding on this genetic variation it is 

 necessary to consider how genetic variation is affected by the absence of 

 inbreeding, namely random mating. 



Random Mating 



Taking the extreme case of an indefinitely large random breeding popula- 

 tion, undisturbed by such factors as mutation, it has been shown theoret- 

 ically that, whatever the original proportions of any two alleles {A, a) may 

 be, the proportions of the heterozygous {Ad) and the two homozygous {A A 

 and ad) classes of zygotes reach an equilibrium in not more than two 

 generations. Further, the relative frequencies of all possible genotypes 

 {AABbcc . . . , AabbCc . . . , etc.) tend to approach an equilibrium in 

 which the different series of genes are combined at random. Linkage has 

 no effect on the ultimate equilibrium. With reversible or irreversible muta- 

 tions occurring at constant rates there will be an approach to a new 

 equilibrium. 



In practice, the above conditions are not found. Such factors as selec- 

 tion and limited size of population will change the relative frequencies of the 

 various genotypes from generation to generation. Provided none of these 

 factors is intensive, however, considerable genetic variation will remain. 

 We can now consider what effect more or less intensive degrees of inbreed- 

 ing will have on that variation. 



Inbreeding 



The primary effect of all systems of inbreeding is an increase in the 

 proportion of homozygous gene pairs present in the population. With some 



