INHERITANCE 757 



Sonneborn, six showed no change of type at endomixis. All these six 

 were of mating type I. 



Such clones may be designated as single-type clones, as compared with 

 the more common double-type clones, in which from a single clone both 

 sexes are produced at endomixis. 



Crosses between single-type and double-type clones. — Crosses between 

 the two types (Sonneborn, 1939) show that they are inherited in typical 

 Mendelian fashion, double type being dominant over single type. The 

 factor for double type may therefore be designated A, that for single 

 type a. The two original diploid clones are AA and aa. Crosses of the 

 two (AA X 2.a) gave in the 149 pairs examined, all double-type (Aa) 

 offspring. Mating together these heterozygotes yielded, in 120 pairs, 88 

 of the dominant double type, 32 of the recessive single type, so that the 

 results approximate the typical three-to-one ratio (Aa X -^3. = A A -|- 

 2 A -|- aa) . The hybrids Aa were back crossed to the single-type parents 

 (aa) in 165 pairs; these yielded 88 double-type and 77 single-type de- 

 scendant clones, an approximation to the expected one-to-one ratio ( Aa X 

 aa = Aa -|- aa). Here we have diploid Mendelian inheritance, one of 

 the characters being recessive. 



Genetic evidence of autogamy. — In the further study of the heterozy- 

 gotes Aa, a discovery of great interest was made. At endomixis, some 

 of these heterozygotic individuals produce (at the first fission after endo- 

 mixis) double-type clones, others single-type clones. Is the double-type 

 clone the heterozygote Aa or the homozygote AA? This was tested by 

 mating them with the normal single-type individuals aa. All the de- 

 scendant clones are the heterozygotic double-type Aa. This shows that 

 the double-type clones produced at endomixis are the homozygotes A A; 

 correspondingly, the single-type clones produced at endomixis are neces- 

 sarily aa. From the heterozygotes Aa there are produced at endomixis two 

 types, both homozygotic: A A and aa. 



How is this result brought about? It is the natural consequence of the 

 occurrence at endomixis of a reduction division with subsequent union 

 of two of the reduced nuclei (autogamy), as described cytologically by 

 Diller (1936). The heterozygote nucleus before reduction is Aa. By re- 

 duction are produced haploid nuclei A and a: by a second division these 

 giwQ rise to four nuclei A, A, a, a. Three of these haploid nuclei de- 

 generate, leaving but one, A or a. This remaining nucleus now under- 



