760 



INHERITANCE 



cases; to B or C in few cases. In about half the cases they remain un- 

 changed. Parallel statements may be made for the individuals originally 

 of type D. 



It is clear that although all four mating types are present in the de- 

 scendants of a cross of but two types, the statistical make-up of the de- 

 scendant population is influenced by the constitution of the two parents. 



Table 19: Mating Types of Descendant Clones from 131 Pairs 

 (Including 279 Descendant Clones), in the Six Possible Crosses 

 OF the Four Mating Types of Variety 1, Paramecium bursaria 



The majority of the descendant clones belong to one or the other of the 

 two parental types. This is evident in the results of all the crosses that 

 have been made, as shown in Table 19. 



As Table 19 shows, of 131 pairs in which the original type of the 

 conjugants was known, 11 6, or 88.5 percent, gave descendant clones that 

 were of the same type as one or the other of the two parent individuals, 

 while but 15, or 11.5 percent, gave descendants that were of different 

 mating type from either of the two parents. 



What determines the type to which a particular clone belongs? In all 

 the 131 pairs of Table 19, all surviving clones from any pair (1 to 4 

 clones per pair, in different cases) were of the same mating type. The 

 four clones descended from the two members of any one pair were culti- 

 vated separately and tested separately. The fact that all four turn out to 

 be of the same mating type shows that the type is determined at the 

 time that the two ex-conjugants separate and before they divide. It might 

 be determined at that time either by internal or external conditions. The 

 fact that in the great majority of cases the descendant clones are of the 



