INBRED AND HYBRID ANIMALS AND THEIR VALUE 341 



agouti. There are, however, other cases which do not depend on this simple 

 cause. Wright's thorough analysis of Polydactyly in the guinea pig (20) 

 again provides a good example. A cross between a three-toed and a four- 

 toed strain gave all three-toed, yet Wright was able to show that at least 

 four, probably more, genes were involved and that there was no evidence 

 of dominance. 



In almost any character connected with vigor or fertility the average in the 

 hybrid commonly exceeds both parents (14), hence the term "hybrid vigor." 



Variation. — It is. frequently assumed that, because of their comparable 

 genetic uniformity, the parental strains and the Fi will have the same 

 degree of phenotypic variation. This is a common result, but it is not the 

 only one possible. We have already shown that the extent of the variation 

 in a character in an inbred line is determined by the nature of the genotype 

 fixed, as well as by the absence of genetic variation, for the effect of environ- 

 mental variation may differ with different genotypes. Similarly, pheno- 

 typic variation in a hybrid is dependent on the genotype of the hybrid 

 and may be less or greater than that in the parental strains. 



Differences between reciprocal hybrids. — The following three factors 

 may cause a phenotypic difference between reciprocal hybrids. The dif- 

 ference may be one of average, variation or both. 



1. The opposite origin of the X and Y chromosomes in the two hybrids 

 may result in a phenotypic difference in the heterogametic sex. 



2. The cytoplasm contributed by the mother may differ in reciprocal 

 crosses as a result of gene action in the female germ cells prior to fertilization. 



J. The "biological environments" in which the two hybrids develop 

 may differ. In mammals the most important influence of this type is 

 probably the maternal environment. A difference attributable to a 

 maternal effect may have had its origin before fertilization, between fer- 

 tilization and birth, or even postnatally. 



Differences in reciprocal hybrids have been recorded, but the total 

 number is not large. The present author would predict that they will be 

 observed more commonly as geneticists turn their attention away from 

 characters that are not susceptible to environmental variation and hence 

 not subject to the effect of the third factor given above. 



The Value of Hybrids in Research 



Compared with Inbred Lines 



Allowing for the distinction between reciprocal crosses, Fi hybrids and 

 inbred lines have the same degree of genetic uniformity and comparable 



