VARIATION AND HEREDITY . 185 



(1) Stocks containing two mating types. When types I and II 

 conjugate, among a set of exconjugants some produce all of one 

 mating type, others all of the other mating type and still others 

 both types (one of one type and the other of the other type) . In the 

 last mentioned exconjugants, the types segregate usually at the 

 first division, since of the two individuals produced by the first divi- 

 sion, one and all its progeny, are of one mating type, and the other 

 and all its progeny are of the other mating type. A similar change 

 was also found to take place at autogamy. Sonneborn considers 

 that the mating types are determined by macronuclei, as judged by 

 segregation at first or sometimes second division in exconjugants and 

 by the influence of temperature during conjugation and the first 

 division. 



(2) Stocks containing only one mating type. No conjugation oc- 

 curs in such stocks. Autogamy does not produce any change in type 

 which is always type I. Stocks that contain type II only have not 

 yet been found. 



(3) Hybrids between stocks containing one and two mating types. 

 When the members of the stock containing both types I and II 

 (two-type condition) conjugate with those of the stock containing 

 one type (one-type condition), all the descendants of the hybrid 

 exconjugants show two-type condition, which shows the dominancy 

 of two-type condition over one-type condition. The factor for the 

 two-type condition may be designated A and that for the one-type 

 condition a. The parent stocks are AA and aa, and all Fi hybrids Aa. 

 When the hybrids (Aa) are backcrossed to recessive parent (aa) 

 (158 conjugating pairs in one experiment), approximately one-half 

 (81) of the pairs give rise to two-type condition (Aa) and the remain- 

 ing one-half (77) of the pairs to one-type condition (aa), thus showing 

 a typical Mendelian result. When Fi hybrids (Aa) were interbred by 

 120 conjugating pairs, each exconjugant in 88 of the pairs gave 

 rise to two-type condition and each exconjugant in 32 pairs pro- 

 duced one-t3^pe condition, thus approximating an expected Men- 

 delian ratio of 3 dominants to 1 recessive. That the F2 dominants 

 are composed of two-thirds heterozygotes (Aa) and one-third homo- 

 zygotes (AA) was confirmed by the results obtained by allowing F2 

 dominants to conjugate with the recessive parent stock (aa). Of 19 

 pairs of conjugants, 6 pairs gave rise to only dominant progeny, 

 which shows that they were homoz^^gous (AA) and their progeny 

 heterozygous (Aa), while 13 pairs produced one-half dominants and 

 one-half recessives, which indicates that they were heterozygous 

 (Aa) and their progeny half homozygous (aa) and half heterozygous 



