INHERITANCE 759 



as indicated in the diagram of Figure 175. After the attainment of sexual 

 maturity, the mating type of each of the four clones is discovered by 

 testing them with standard clones representing the four known mating 

 types. 



The fullest data as yet available are for the cross A X D. Of this cross 

 the mating types have been determined for the clones descended from 

 61 pairs, including 175 clones. The 4 original clones did not survive in 

 all the pairs. They did all survive in 26 pairs (104 clones). The remain- 

 ing pairs had each but 1, 2, or 3 surviving clones. But in all cases all 



Table 18: Inheritance of Mating Types (the Four Types Are 

 A, B, C, D), Paramecium bursaria 



clones descended from any pair were of the same mating type. The re- 

 sults have been summarized as in Table 18. 



In Table 18 the following general relations appear: 



1. Ail the descendants of any one pair are of the same mating type, 

 though the parents were of two different types. 



2. Among the descendants of the cross of the two types A and D 

 occur all four types A, B, C, and D. 



3. The majority of the descendant clones are like one or the other of 

 the two parents in type; only a few differ. In this case 53 of the 61 

 clones are either A or D (the parental types) ; only 8 are of types differ- 

 ent from those of the parents. 



4. By conjugation the mating type has become changed in one or 

 both of the ex-con jugants and their descendants. In the above table, the 

 type is changed in both ex-con jugants in 8 pairs out of 61; it is changed 

 in but one of the ex-conjugants in the remaining 53 pairs. 



The change in type is due to the exchange of migratory pronuclei, 

 since the cytoplasm remains unmixed. Individuals of type A, receiving a 

 micronucleus from type D, are changed to type D in nearly half of the 



