204 MULTIPLE ALLELOMORPHS 



that a factor could change in more than one way, 

 and thus give rise to multiple allelomorphs, unless 

 it is supposed that the only change possible in a factor 

 is a complete loss of the factor, as postulated in the 

 presence and absence theory. 



There is, however, an alternative theory to that of 

 multiple allelomorphism. This alternative is com- 

 plete linkage. The numerical result can be equally 

 well explained if, instead of occupying identical loci, 

 the factors are so near together that they never 

 (or very rarely) cross over. For reasons that will be 

 given later we are inclined to think that the 

 explanation of multiple allelomorphism is in most 

 cases the more probable one, but the arguments in 

 favor of this view may be deferred until the facts 

 have been described. 



There is a general relation that so far holds for 

 all cases in which multiple allelomorphs have been 

 discovered, namely, that the factor-differences pro- 

 duce similar effects. All of the following examples 

 illustrate this relation. 



In rabbits (Fig. 50) the Himalayan pattern has 

 been shown to behave as a recessive to self-color and 

 a dominant to albino. Any two of these three types 

 of pigment formation and distribution give a 3:1 

 ratio in F2 but no two of them, when crossed, ever 

 produce the third genetic type. In other words the 

 factors behave as though allelomorphic, for only two 

 can be gotten into any one individual. A similar re- 

 lation has been described by Baur in the columbine, 

 where three types of leaves, green, variegated (green 



