VARIATION AND HEREDITY 235 



Upon this supposition, type I has one allele in common with type II, 

 and this allele is homozygous in type IV. It has one allele in common 

 with type V, and this allele is homozygous in type VI. Type II has 

 one allele in common with type V and this is homozygous in type 

 III. These alleles were designated by Kimball, mt 1 , mt 2 , and mt 3 . 

 The genotypes of the six mating types may be indicated as follows: 

 imVmtMI), rn^mt 3 (II), mt 3 mt 3 (III), mtfmt 1 (IV), mt 2 mt 3 (V), and 

 mt 2 mt 2 (VI). 



There is no dominance among these alleles, the three heterozygous 

 combinations determining three mating types being different from 

 one another and from the three determined by homozygous combi- 

 nation. Kimball (1939, 1941) had shown that the fluid obtained free 

 of Euplotes from a culture of one mating type will induce conjuga- 

 tion among animals of certain other mating types. When all possible 

 combinations of fluids and animals are made, it was found that the 

 fluid from any of the heterozygous types induces conjugation among 

 animals of any types other than its own and the fluid from any of 

 the homozygous types induces conjugation only among animals of 

 the types which do not have the same allele as the type from 

 which the fluid came. These reactions may be explained by an 

 assumption that each of the mating type alleles is responsible for 

 the production by the animal of a specific conjugation-inducing 

 substance. Thus the two alleles in a heterozygote act independently 

 of each other; each brings about the production by the animal of a 

 substance of its own. Thus heterozygous animals are induced to con- 

 jugate only by the fluids from individuals which possess an allele 

 not present in the heterozygotes. 



The double animals of Euplotes patella (p. 228) conjugate with 

 double animals or with single animals in appropriate mixtures and at 

 times a double animal gives rise by binary fission to a double and 

 two single animals instead of two animals (Fig. 98). Powers (1943) 

 obtained doubles of various genotypes for mating types which were 

 determined by observing the mating type of each of the two singles 

 that arose from the doubles. Doubles of type IV (m^mt 1 ) with a 

 single micronucleus (Fig. 98, a) were mated with singles of type VI 

 (mt 2 mt 2 ) (6). The double exconjugants (d) were "split" into their 

 component singles belonging to mating types IV and VI (g), while 

 the doubles were type I (/) . Thus it was found that the phenotype of 

 a double animal with separate nuclei was the same as though the 

 alleles present in the nuclei were located within one nucleus. The 

 fact that loss of one micronucleus had no effect on the type of 

 doubles, tends to show that the micronucleus has no direct effect on 



